Just went to weigh myself out of interest, turns out I made an error: I'm actually 5'10 and 120lb. Sorry for the error, I either weighed more before or just clean forgot.
I appreciate all the replies, it's nice to see my question is being answered. I'd say the ones who pointed out that it's a problem that I have with myself are probably right on the button. I've always been thin as a razor, but it wasn't until I started to get into girls that I had a problem.
I've had a few comments before, things generally like "I like you, but I could never be with you - you look like I'd snap you in two!" and things like that, plus girls who have laughed at me when I said I'd give them a piggyback. Skinny girls, too.
I suppose that doesn't do much for morale, so when I'm approaching I constantly have it in the back of my mind that perhaps she'll not like me because I look like a twig.
Perhaps framework and things like that are beyond my control, but it would be nice not to be able to see the bones in my shoulders and things like that. I'll take a trip over to the fitness section and see what I can rustle up.
Thanks, brosephines!