I leave a dreadful effect on my ex girlfriends! Please help?

[sam s]

So here is my story, i am 24.. and started dating at 16. I have had 3 long term relationships so far. All my 3 relationships ends after 2 years of a great passionate love that's like Romeo and Juliet, but eventually the girls i have dated so far all end up cheating on me and saying the relationship is too serious for our age. They all talk of marriage, and even move away from their families, provinces away to be close to me but still screw me over at the end. I am now worried because after the break ups, all the 3 of them say they are in an state of insanity until today i still get messages, calls from them... and they usually start drinking and smoking. Its hurts me so much... but because i can not take a cheater back it makes my life difficult trying to help them! I am always wondering why do i have such a lasting affection effect that results in damage? Everyone says i am a nice guy, but i think im a good lover not a nice guy. Anyone knows what i can do to break the pattern... im now so paranoid and scared of dating? These girls end up so broken, even though they are the ones that decides to break up for no reason... i am not abusive or violent at all. Please help