I can relate to you as well; I started dating my first ever boyfriend in high school when I was around a size 16/18. By the time I broke it off with him (due to going away for college) I was a solid size 20, approaching a 22. Most of my freshman year, and the summer leading up I absolutely freaked out that no one would be interested in dating me because of my size. My first boyfriend was all due to a mutual friend so I had never before been successful in finding a guy on my own. From September to December, I adamantly chased guys here and there, coming off a bit strong, until I happened to meet one guy from a club I joined. He didn't really work out for a lot of reasons, but I kept dating him because I was convinced I would be alone otherwise.
It's really true what they say, you always find love when you're not looking. This guy was clueless and didn't know how to treat a girl, but then I met my now boyfriend (at the same club) without even actively going after him. In fact, my room mate pointed out that I was flirting with my now boyfriend and that I had better dump my then boyfriend because it was going nowhere fast. I was still nervous but ended the relationship because I didn't really care for him. Then, things moved really fast and I was suddenly asking this new guy out (via text, how lame). He called me and we talked for a few hours and all of a sudden we were a couple.
Now I'm in my Senior year and it's coming up on 3 years for us, and hopefully more as we've been talking engagement. I really do understand your side though, because I was such a defeatist when I thought of actually trying to date. I know it's difficult not to diet for the sake of others, but as hard as it is to keep your mind off of the subject, it's not impossible. Before my first boyfriend, I put my time into constructive things like starting a club at school or marching band rather than pitying my situation. Try some new hobbies, you could meet a guy like that. I actually also have some friends who met their significant others at the gym