Nobody here can guess what the problem is. Also, age is always helpful. If you're 22, there's probably no issue. If you're 35, that's different.
The only thing I can suggest is, if you're older, think carefully about what these guys say when they break it off. There's a pattern in here somewhere, and you're the only one who can identify it. Another option is simply pick one of them you're more comfortable talking to and ask them if they'd be willing to explain in more detail what you're doing wrong. I even know someone who did this.
The other pattern might be you're attracted to the wrong kind of guys.