Seems like you have a problem that needs to be worked at two ends:
1.) you need to change your own tactics of attracting guys... interacting with them
2.) need to have a better idea of guy's own tactics for attracting women... how to tell when a guy is looking for something short term as well as when they are looking for something long term.
I suspect your run of bad luck is from putting out the wrong signals and possibly becoming sexual way too early in the 'relationship.'
PUAff makes some good suggestions about the 'dating game' which would help filter out the short-term guys from the potential long-term guys.
Carl makes a good point as well... how you present yourself to these guys is that you're looking for short-term (aka sex). Then you probably offer sex pretty early, which only encourages these men further.
So yes, more information is needed to give more advice on what you may be doing wrong and how to improve this... so that you can attract a long-term guy... instead of 'accidentally' attracting a short-term guy.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen