I will tell you exactly whats going on. They're reading the situation; each situation, as an opportunity, and unfortunately you fall right into the age old practice of "believing" what these boys are telling you. Look, you want to belong with someone; nothing wrong with that. Where you go wrong is you put too much skin(pardon the pun) in the deal, and all bets are off as soon as they get what they want.
Some guys just hit and skip, others stick around long enough to be annoyed by the infatuation; and still the other kind, might actually find greater interest in what you have to offer besides your body, and those are the ones, not all of them, just one, that you should save your body for. Stop giving your love away to "sweet nothings" The love you save, might just be your own.
You've got to "up" your worth, I'm certainly not saying you aren't worth anything, I'm only saying, evaluate what you attract. If you meet these men in bars, stop doing that. Meet one at a book store or coffee shop, the wrong ones might there, too...but you HAVE GOT TO try different things...don't rush to fall for a guy, give it some time...appreciate your single-hood...and I guess one other thing...stop looking..let it find you, develop you.