I think there are moments (I hope) that really attractive girls doubt themselves too. Girls are just naturally neurotic people, I reckon.
But from your posts and stuff, Ash, you sound like a rally funny and intelligent person. Relationships are kinda like things you lose, when you're not looking for them, they pop up. Try not to think about it too much, which I know sounds stupid and is hard. But the whole weight thing...I mean, it's the only body you have, so you should get comfortable with it. If you're not happy, try and lose weight. But don't do it for anyone else, or in the hope anyone else would like you when you're thin. Do it for yourself.
There's no sure-fire way of getting a guy, but if you're worried about weight, maybe start out as friends first, and work it out like that...? But then there are a whole set of other problems...
It'll be ok. Relationships are beset with obstacles, at best.