Hi. I don't know if it helps to say this, but I will anyway.
I am quite old, and have found throughout my life that people can be attracted to all sorts of body types. I know personally one couple made up of a large girl and a guy who goes to the gym a lot, is in the military and is in extremely good shape. To look at him, you'd never think he'd give a larger girl the time of day. Yet, they're getting married this year. They're both extremely sweet people, and I'm happy for them.
I myself am not attractive to the majority of people. Yet, I have found that when a person likes me, they REALLY like me. And I think that this is spurred by them liking my personality, and their physical attraction to me flows from that, rather the than the other way around.
My girlfriend doesn't consider herself attractive, but she is literally the most beautiful person in the world. It's like she was made specifically to be atrractive to me. Every single trait, even things she doesn't like about herself, I adore. But if you'd have asked her what she thinks of herself, you'd have gotten a severe underrating.
I suppose what I'm trying to say is, don't give up. Try not to compare yourself with others - I KNOW, from myself, that this is easier said than done. But, try.
Focus not on being the best girl, but on being the best funeralcat that you can be, each day. Don't worry about perfect. Accept yourself as you are and take small but consistent, persistent actions towards YOUR (not society's) goals. Those goals can be whatever you want. Keeping the topic on feeling attractive - say you don't feel like doing anything to do with losing weight this week. Well, maybe you decide you're going to look at haircare, or something, instead. Do something that's fun to you and you'll make progress, show yourself and others that you value yourself, and maybe be able to snowball to other things as you build confidence.
Those guys that chase your friends are doing you a favour. You don't want those guys. You want the guys that like YOU. Perhaps that's a smaller number than the guys that like other girls. That's a bit brutal to know and I empathise, I've been there. But at the end of the day, there ARE guys out there that will like you, for you, right now, with no extra effort required.
If you can go the extra mile, and let go of jealousy. Be the best damn wingman there is. Help the other girls stay safe and happy. The more people that love you, the more you socialise, the wider the net is. The wider the net is, the more likely you are to find the people that like YOU. I don't know if this applies to women but for men I always say, don't sweat it if a girl doesn't like you like that. Just be patient and kind. That girl who doesn't like you might have a friend who does.
Something I probably shouldn't say. But if you don't know, you should know. There are guys that specifically like bigger girls, even to the point of having a fetish for it; it's really far from rare. Check numbers on any porn site if you would like proof. I imagine you don't want to be fetishised, and that's understandable. As a guy, I have less problem being someone's fetish. But anyway, maybe that's a better feeling than being nobody's fetish - I hope so, otherwise, blank this paragraph from your mind, lol.
Wishing you well. Don't lose hope over this.
One last thing. Get off instagram, or change your feed to be less noxious - it's harming you like it does so many people. Even if you want to make these things your goal, messing up your mentality like that won't help you get there. If it's a self-harm thing, try to channel that desire into something positive - pain from lifting heavy weights, or something. That's not gender-specific advice, either, you won't become Arnold Swartzenegger (ridiculously big muscles) without steroids, trust me.
Alright I'm talking real nonsense now, I'll leave it there. Good luck :)