Most Helpful Guy
Maybe you're getting the wrong read on the guys you're going after. I notice in one of your comments you mention that you try and go after "good guys," but the outcome sounds more like these guys are looking for a casual relationship, possibly for the easy sex. That they have another girl lined up to replace you would seem to support that.
In an opinion I gave yesterday I noted that even among "nice guys" there are some who don't really want a serious relationship but see niceness as their best route to sex. Had they more going for them and better game, they'd prefer to be players, but they'll take what they can get. It may be that, for some reason, your guy radar draws you to guys that fall into that category.
Alternatively, it may be that you, knowing what you want, are continually (even if subconsciously) trying to take the relationship to the next level, and you're working with a pool of guys who are just not interested in going that far yet. Maybe they feel like you're moving things too fast for them (and why they might feel they're holding you back), and they'd rather be in a situation where they feel they have more flexibility for now.
Anyhow, you seem like a good woman with her heart and mind in the right place. Hopefully you'll find a guy that feels he's good enough for you in the not too distant future.