How to overcome insecurity?

I am insecure about pretty much everything, from the way I look to the way I sound and my abilities. The last time I remember being secure in myself was when I was 11, as I didn't care about appearance, and I'm 18 now.
I'm in a relationship that's soon to go long distance (uni whilst he is in year 13) and I'm worrying about everything. I worry he'll fall for another girl, especially a younger, prettier girl in year 12 as the school is really pushing them to be good friends between the years. I worry he'll decide I'm not worth the effort. I worry about everything and it's causing tension and arguments. He's not given me a reason to worry, it's just all in my head.
How can I overcome such deeply ingrained insecurity? I have nothing that I like about myself at all and I suffer from anxiety, so it's very difficult for me and I really need some good tips.

(also, as a bonus, can you recommend some hobbies and things to do whilst he's busy? He's going away for 3 weeks and I have nothing to do to keep myself entertained and not worry about the pretty girls on the beach in their bikinis).

I am insecure about pretty much everything, from the way I look to the way I sound and my abilities. The last time I remember being secure in myself was when I was 11, as I didn't care about appearance, and I'm 18 now.
I'm in a relationship that's soon to go long distance (uni whilst he is in year 13) and I'm worrying about everything. I worry he'll fall for another girl, especially a younger, prettier girl in year 12 as the school is really pushing them to be good friends between the years. I worry he'll decide I'm not worth the effort. I worry about everything and it's causing tension and arguments. He's not given me a reason to worry, it's just all in my head.
How can I overcome such deeply ingrained insecurity? I have nothing that I like about myself at all and I suffer from anxiety, so it's very difficult for me and I really need some good tips.

(also, as a bonus, can you recommend some hobbies and things to do whilst he's busy? He's going away for 3 weeks and I have nothing to do to keep myself entertained and not worry about the pretty girls on the beach in their bikinis).

General insecurity takes time to work on. Insecurity involving your relationship can be dealt with by communicating with your partner and getting support and reassurance from him. If you don't receive that from him then it isn't ideal and you may not be with the right person.
You could look up some self help guides on anxiety which help to analyse your thought processes and categorise and rationalise them. Often by doing a task like this you begin to acknowledge a different perspective and a different world which isn't clouded by your insecurities. There are a lot of free resources online.
Communicate with people close to you who are likely to encourage you. Praise is helpful, because it enables you to see another perspective. Hopefully you have people like that in your life who you trust.

Hobbies depends on what you like doing. I would say stuff like cooking, baking, yoga, exercise etc, all of which have some kind of therapeutic value and can release endorphins.

He reassures me all the time that I have nothing to worry about and that I'm beautiful. But I just have this thing in my head that assumes that, because he's with these girls in his class and year and they go on fun school trips, that I don't have any memories to live up to the ones he has with his friends and these girls. I don't know how to get over this jealousy surrounding them seeing him more and him having fun with them (I think it centers around the fact I don't think So as we can see fun to be around). I just really don't know how to be positive or not care that he's surrounded by beautiful girls that (I think, I don't know if other people do but I assume they do) are funnier and he has more in common with. Plus they're skinny and I've always wanted to be skinny but I don't have the willpower
I know how stupid I sound, and I know this- hence why I'm asking for help! I'm just so used to not being worth anything
I love cooking, but I'm trying to lose weight currently and I'd just eat it so that's probably not the best idea sorry I just went on a massive feeling splurge on you there

So as we can see, there are a lot of insecurities here. This is where I repeat my advice to get self help resources online. That way you can go through these thoughts one by one.

Im confused by your point on cooking. There is nothing wrong with cooking and eating! As long as it's healthy. You can treat yourself by baking but it's obviously about making sure it is in moderation. There are plenty of healthy recipes out there.

I love to bake, but I don't like to cook because I'm not very good at it. Do you happen to have any good resources you know about?