What is happening, I keep getting hurt!! Do I deserve this, will it work out? Please Help?

Hi. I'm 27 years old. Five years ago, i got with somebody, who was pregnant. I never knew until she did, and we were going out nearly 3 months then. I stuck by her and eventually, after 3 years, she cheated on me, hurt me really badly, and left me. I continued to see my son, even though biologically, he was not mine. Eventually i got over her, and two years later, met someone who changed my world, i fell head over heels in love with her. She has a 4 year old daughter. Anyway, she had been hurt in the same way i had by her ex, however she was married so was worse. To cut a long story short, i cut out my son as i belived in the long run, it would benefit him and me, especially if i have my own family. However, this girl has cheated on me already, twice, although only kissing someone else. She constantly hurts me, and although admits she loves me a little bit, last night told me she is not happy and we should split. I do love her, and her daughter so much, i've done so much to make her happy, is this going to be a case of don't know what you have got until its gone?