Why do men give so many mixed signals?

All of them? Well then there is obviously something wrong , either with your behavior or whatever. It's difficult to say what it is without knowing you or knowing how you talk to those guys etc.

Well how do you try to continue things? do you come on as too desperate because that's what it sounds like..

Maybe you come on too strong? You sound as though you're doing all the work, let the guys take the initiative more. If you leave them alone, you'll see who really wants to be with you because they'll do the work to get to know you.
Screw all the guys who don't do anything, maybe they have a crush on some other girl and don't care about anyone else or for some other reason they just don't like you. Let them go (and don't keep any attractive guy friends as 'potential boyfriend', because they just like your attention and use it).
Let them come to you, not the other way around.
And when they come, don't get all excited. Act cool, you're an interesting girl and they should put in some effort if they want you to like them.

I'm with BananaPeelz. Think about it for a second... ALL these guys are doing the same thing, now what's the common denominator between ALL of them? You. So it's either that you're doing something wrong or you're going after the wrong kind of guys.