I think part of a relationship is the feeling that someone truly cares for you, and when you see people who are in great relationships, and you see that you can't seem to get into a relationship, you can feel unwanted, unaccepted and not good enough. You start to pick at your perceived flaws even more trying to work out 'what is it about me that is so unappealing?' and then get even more down on yourself. I am like that, and sometimes I REALLY wonder how certain people can get in a relationship and I can't. That makes me feel even worse.
In regards to your weight, and guys not liking you because of that, I have a question- would you really want to be with someone who is only going to like you because you are thin? Or would you rather be with a guy who likes you because he likes YOU?
I had a male friend who told me I'd be more attractive if I 'put the effort in', and 'toned up a bit'. More attractive to WHO? I'd be more attractive to HIM. And him being one of the most superficial people I've ever met, why would I want to lose weight and make myself more appealing to HIS idealisations of what attractive is? If you want to lose weight do it for you, not someone else.