How does your relationships always end? I don't really know if you are referring to dating the same breed of men? or you have issues and that is why you keep having the same end result? I want to help you but I am still having a hard time trying to figure out what you mean by "I know better than to get myself into a relationship because I know how it always ends"
For example... if you keep dating the kind of men that are players - you will get similar results...or if you have insecurity issues and let's say you interrogate all the boys you date - yes, most likely it will yield the same result. I AM NOT SAYING YOU ARE DOING THIS. I dont even know what it is, I am just giving you an example...
I dont know if you feel comfortable sharing what it is thats causing your relationships to turn out the way they do... but its something that is obviously reoccurring, now unless you change whatever that is or figure out what it is... you will probably continue to face the same issues/results
You should not give up on relationships!!! I hope you open up some more that way I can give you better advice