I can relate to this on a very personal level. I sort of went through a similar thing myself, but the most important thing is to build up your self esteem again. Every time you sleep with a new guy for the sake of those 2 hours of happiness, you're actually chipping away at your self worth and esteem. At the end of the day, you know that those guys don't care about you and are just in it for the sex and the more you do it, the more you'll start to believe that you're not worth loving. Believe me when I say that you are though. You are worth loving just as much as anybody else, but unless you believe that and start reducing the number of casual encounters you have and more actively concentrate on focusing on guys you could actually potentially have a long term future with, you're only destroying yourself bit by bit.
For me it took a guy waiting 6 weeks for me to have sex with him to make me realise that I was still worth caring for, for you maybe a different experience will open your way into a new mindset. It is your mindset though that you need to work on. You need to re-channel yourself into a state where you believe that a relationship is what you deserve and that any less isn't worth it in the long run. It's not easy, you'll probably fall back from time to time and have doubts, but in the end once you dig yourself out of this detrimental cycle you'll be a lot better off. Take this from someone who knows... and is battling that fight day by day.
Also, I very much recommend the following books if you're into reading at all. I found they helped me get a better idea of what I am actually looking for in a potential relationship, and also look out for those guys providing all that I don't need in a relationship too. They may not hold the answer for you, but then again they might.
Greg Behrendt - He's Just Not That Into You + It's Just a Date: A Guide to a Sane Dating Life
Steve Harvey - Act Like a Lady, Think Like a Man
Meg Jay - The Defining Decade: Why Your Twenties Matter and How To Make the Most of them Now
I just hope that you can break the cycle and work things out, because I know that lost and lonely feeling and I don't wish it on anybody. I believe it's true that you need to believe in yourself and your own happiness before you can make someone else truly happy, so I think you will need to start finding ways of making yourself happy without the help of meaningless sex before you will be able to find something more lasting and concrete. Good luck with everything and feel free to PM if you feel the need.