I'm wondering why your relationships don't last... Do you end it, or do they? What are the complaints? What are yours?
That very well could hide your answer
(It's a bit of a stretch, but from experience I've seen this work for others; analyzing why relationships themselves failed sometimes helps a bit when understanding your appeal as a person, for better or worse)
PS: It is true that you should never change yourself for someone else (indeed!), but you can certainly change perspective. By this I mean: What are you looking for in a partner? What do you want from someone? How do tou want them to treat you?
Changing these, broadening them, or narrowing them down, or maybe even altering some of them (just as examples: I find that many of us are very picky, and overlook good guys because of weird things they do to turn us away. Or sometimes, we settle for guys that don't treat us well, to go for a "sexy personality". Anything of that sort). My advice may sound silly to others, but the moment I changed my "what I want in a guy" vision and rearranged my priority list to something more broad and more based on his character, I found a perfect match :]
Turns out I'd been looking for the wrong kind of personality (and style, since those two are often related) to match mine >_> lol