Positive
Well done for completing your first year of Uni and it is lovely that you really enjoyed it. Please hold onto those positive and good thoughts because you know absolutely that 'Yes you can do this' You are clearly articulate and educated enough to get a 2:1
Positive
You have two good friends so treasure them. They are gold dust. Those other people you mention, with apparently hundreds of friends all laughing smiling having a good time (& btw this is often an illusion) only have less than a handful of very very good close, genuine caring friends. Friends you would lay down your life for. Remember that and take time to nurture those people closest to you who are your cheer leaders.
Positive
You have a caring family and siblings who are rooting for you to succeed
Negative
I am sensing so much of 'poor me' 'help me' sentences in your post as if you are used to finding a problem and then being 'helpless' to do anything about it?
It seems you have listened far too much to friends and some family, and you have lost the ability to know your own mind? Other people are unduly influencing you and controlling your decisions. You swap and change your own mind on a whim like the wind changing instead of thinking things through thoroughly and carefully? Clearly something has happened between finishing Uni and enjoying it and then suddenly going into a tail spin on a field trip? What really went wrong for you?
There is so much self doubt in your post
1) You will get lonely
2) You won't be able to do the maths
3) You are not good enough (no intelligence, no ability)
4) I can't do maths
5) I don't think I'm capable enough for this
6) I'm not part of my old friendship group
7) The girl I thought I should share with (because I didn't want to get lonely) isn't very nice
8) My family tell me the girl I share with isn't very nice
9) My parents want me to give up and come home (so I can be even more unhappy at home)
In your head every decision is a drama, and yes you are being dramatic. If this is the logical calm thinking in your head right now no wonder you feel so unhappy. Your head must be crucified, and even reading your post I can feel my own brain start rebelling. You have a major decision to make. Go home and never grow up, ensuring you let other people make all the decisions for you. You have already said you are unhappy at home so how will that help you? Or stay and take time to get all the available information and help that you can from University; from your Uni medical centre, your own health care provider and see if you can get some help. Cognitive Behavioural Therapy (CBT) could help to change the negative self doubt narrative that is etched into your head. Sometimes years of conditioning of being allowed to think negatively and being controlled can become a normal dysfunctional way of thinking for you in the every day and for life. Change the narrative in your head. Be aware of negative thoughts. Stop them.
The following are words of advice but you have to take every single part of this information and the information you gather and research it all before you swap and change. You have and make a list of your options. Options in terms of pro's and con's; what you love, what you hate, what the time scales are, how much additional finance will you need, can you fund it, could I get a part time job. It's ok to make a bad decision, and to fail. But if you research thoroughly you will make less bad decisions, and even if you do happen to fail you will have done your best and be ok with it. Own decisions and take responsibility for them but only if you have researched, asked questions, know the facts and then look at +ve and -ve.
The big word missing here is DETERMINATION. Be determined that no one is going to talk you out of what you were so passionate about or talk you out of what your long term goal is. What do you see yourself doing? Working in the UK, or abroad, or in the wet, in the marine environment, in National Parks? Having a passion for plants, of wildlife, for animals, for geography, for the biology of life is amazing. I can't think of a better outlook.
Physical and Mental Health
It is always easy to use a melt down or an eating disorder as a reason not to do something. It is always easier to give up and stop trying than to persevere and suddenly realise that if you really put your mind to do something you can do virtually anything (albeit not overnight) Speak to some medical professional and get advice to improve your state of mind and deal with the eating disorder you state you have. If you have a health issue that stops you maxxing out your enjoyment in life get help sooner rather than later. When you respect your body, give it plenty of sleep, nutrients and exercise it can do wonderful things. If you hate your body, self harm and despise your looks, your life you won't have the Ferrari to drive down the road in comfort. Did your ed affect your ability to be physically robust enough to walk through bogs, marshes and hills for 8 hours? Is that why you couldn't finish your 2 week field trip? Get fit, get healthy, and face your fears head on.
Choice of course
You have been lucky to have a Uni that has allowed you to change course. They sound supportive. Maybe the words of your 'advisor' and head of course have been brutal but truthful? Did you know the field trip would be an integral part of ecology and conservation when you signed up for your course? If that is your passion stick with it. Go back to course admin and find someone to talk this through. Find out what your 1st year ecology/conservation options are. Decide what is really the most important thing that matters to you right now? Your 2 friends or your current choice of course. Both will last a lifetime but if you decide to study you do that for a reason. If you have found 2 really lovely genuine friends you can do that all over again if needs be. You will always have your loyal friends near or far. A new layer of really lovely people. Weigh up the costs. If you are so determined to complete your course you can sofa surf and use the Uni facilities (showers, laundry etc) and ask if anyone can 'box up' meals for you?
Self Doubt
The dreaded Maths and Statistics are involved in the ecology etc. Is that your biggest fear? Talked yourself out of it? You need to basic classes and conquer the maths. Find out the type of maths you will need to undertake for the course content from the tutors. Find remedial classes in maths at Uni or college to become numerically literate and to know how to use those stats and numbers in your course work. This you can do but you have to try first and not give up just because it seems a little bit difficult. Of course maths looks overwhelming if you let it swamp you. Start at the simple beginnings, and get learning times tables, addition, subtraction and then basic maths. Virtually everyone aged 30 years and over in the UK who went to state school is rubbish at maths as no one taught those subjects intensively or well. You are so very lucky if you had an ace maths tutor. If you have never been taught those skills you don't know to do them. It's like trying to read Japanese in English. Then you can see if you can do 'little bits' of learning - then you can achieve the whole. It might not be such a bad decision to take a year out, if your mental health is so bad, and have intensive maths lessons in-between. Then go back to Uni armed to successfully complete your course? Only you know how bad things are. Your health is for a lifetime. You can study at any stage in your life. Study is not the be all and end all.
Accommodation
Go back to the Uni housing office and take advice. Stick it out if it is for one year or look for an urgent swap. Next year you will be wiser, older. Sometimes singletons looking for a house share in year 2 are the argumentative souls no one else wants (but that doesn't mean you can't get on with them like a house on fire) Research, weigh up pro's and con's, likes and dislikes, then make YOUR decision to change or to stay. Ignore others who will tell you what to do. This is your personal choice.
Get into a habit of looking at a problem from all angles. Get your siblings on board. Make your own mind up from your gut feelings and from your intensive research. Ask questions, lots of them. Be brave enough to stand by those decisions. Your siblings clearly want to help you in a positive way. You have to list those pro's and con's and look at all the practical problems. Cut out the negative voices in your head, and those negative voices sat on your shoulder.
Change the can't to CAN and you will open so many doors you never knew were there. Good luck.