I hate my longstanding loneliness

Just because the girl is a flatmate doesn't mean she's an actual 'mate' of yours. She's someone with whom you share a house; sometimes these situations develop into friendships, but she doesn't have to include you in her social life.
What's the issue with your own friends; why aren't you included in activities with them?

I am rarely invited by friends to occasions. I either have to initiate it or insert myself into it. The loneliness depresses me a lot. I used to have flatmates who invited me to join stuff and we had dinners at the student hall canteen a lot. I am living privately now after the end of my postgrad. I only hanged out with my flatmate once. She almost never invited me to parties held in the house. I often came home bumping into a group of strangers and had to hide in my room while being disturbed by all the noises. I once expressed upset about the exclusion and it caused weeks of tension. I got blamed for feeling that way and made to look like a creep for wanting to be included. I don't understand what the issue is. Am I not allowed to feel bad? Am I not allowed to my peace of mind or happiness in where I pay a lot to live? I hate my emotions being invalidated. I hate having to spend nights rolling in my bed with tears while no one else literally bothers to check in on me. I hate the saying that "we are not responsible for others' emotions" when their behaviour is upsetting others. Ain't I right?

I am rarely invited by friends to occasions. I either have to initiate it or insert myself into it. The loneliness depresses me a lot. I used to have flatmates who invited me to join stuff and we had dinners at the student hall canteen a lot. I am living privately now after the end of my postgrad. I only hanged out with my flatmate once. She almost never invited me to parties held in the house. I often came home bumping into a group of strangers and had to hide in my room while being disturbed by all the noises. I once expressed upset about the exclusion and it caused weeks of tension. I got blamed for feeling that way and made to look like a creep for wanting to be included. I don't understand what the issue is. Am I not allowed to feel bad? Am I not allowed to my peace of mind or happiness in where I pay a lot to live? I hate my emotions being invalidated. I hate having to spend nights rolling in my bed with tears while no one else literally bothers to check in on me. I hate the saying that "we are not responsible for others' emotions" when their behaviour is upsetting others. Ain't I right?

Hi, it sounds like your really depressed, what you need is to make new, better friends since is sounds like your current ones aren't any good. Make friends with someone who wont ignore how you feel and will include you in things. It might be hard - it was for me - but try to talk to people you relate with and over time you will grow a stronger friendship.
Just remember, its a bad day, not a bad life.

Only if the person owns the house and is the sole occupant. If not, it is unacceptable to ignore the wellbeing of other occupants.

If she doesn't own the house she's a tenant, same as you, and you aren't her responsibility, and neither is she yours. Why does she have to look out for your wellbeing; what do you do for her?

I am very sorry to hear your position, and I too went through a period that could have been me speaking like you are, seriously. Have you ever wondered if you have a Neurodivergence condition ?. I am thinking perhaps autism, and that as many levels. Even Elon Musk is Autistic so is nothing to be ashamed about ,I think lots of people have it and dont know.
In any event you may like to try Hypnotherapy,which may well help you. Choose a good one and will be worth the £60 an hour.

Are you serious? How is it not each other's responsibility to consider mutual wellbeing? Do you know what a house share is? Is your worldview so warped that by sharing a house you can do whatever you wish to endanger the physical or mental health of others, e.g. trashing the premise, not cleaning up broken glasses, exposing others to allergens to which they may have life-threatening reactions, disrupting people's sleep, denying them a sense of comfort and/or peace in where they pay to stay in etc.?
Not taking that far - the living room is also my living space. How can guests have more privileges than those who pay a significant amount of rent every month? How can tenants entitled to use the living space be made to give way to those contributing nothing to the property?
Have you ever really shared a house with others or spent your life in your care-free parents' house?

Hi, it sounds like your really depressed, what you need is to make new, better friends since is sounds like your current ones aren't any good. Make friends with someone who wont ignore how you feel and will include you in things. It might be hard - it was for me - but try to talk to people you relate with and over time you will grow a stronger friendship.
Just remember, its a bad day, not a bad life.

I think it all depends on perspective.
If I was to have a flatmate that I did not particularly appreciate (which I imagine is the case for your flatmate since she is behaving this way) I would be polite ie: not disturb their sleep/keep my side of house clean/clean my dishes and not leave my clothes lying around etc.. That would be my responsibility, ask permission before inviting people over. BUT I would not consider it MY responsibility to look after the wellbeing of someone unless they are a friend. If I don't like you, I am going to be distant but polite. So I would never come over to your room to check if you are feeling well or not, and if you granted permission to have ppl over, that wouldn't mean you are invited.

It appears that you don't understand what wellbeing is.

"wellbeing
: the state of being happy, healthy, or prosperous"
^This. i don't care if you are happy, healthy or prosperous if you are my flatmate and I don't like you. I care if you are my friend.

So you would trash your place, leave broken glasses around, expose them to allergens they can't handle and blast music all over the house at 4 am?

no, since I said in my previous post I would ask if it is okay to invite ppl over/ keep my part of the house tidy and not put people at risk obviously. But your mental health/social life doesn't interest me if I hold a grudge against you for whatever reason. This is what I meant by wellbeing - not your physical wellbeing. Anyways, I think this conversation is not getting anywhere (at least with me). I hope you get out of your depression or whatever is causing you to feel so down about everything and you can find a hopefully cheaper place that doesn't have nasty flatmates. You sound like you need change in your life (moving to a cheaper area/being exposed to new ppl) but you are not in the headspace to handle that change which can be challenging. Sometimes getting away from places that carry bad memories is the best way to get rid of a vicious circle. Because you can't change OTHER ppl.

Are you serious? How is it not each other's responsibility to consider mutual wellbeing? Do you know what a house share is? Is your worldview so warped that by sharing a house you can do whatever you wish to endanger the physical or mental health of others, e.g. trashing the premise, not cleaning up broken glasses, exposing others to allergens to which they may have life-threatening reactions, disrupting people's sleep, denying them a sense of comfort and/or peace in where they pay to stay in etc.?
Not taking that far - the living room is also my living space. How can guests have more privileges than those who pay a significant amount of rent every month? How can tenants entitled to use the living space be made to give way to those contributing nothing to the property?
Have you ever really shared a house with others or spent your life in your care-free parents' house?

Where is possible?