I had the same experience: I got my first GF at 15 and she was 14. We dated for years while we still lived in the same town. I knew she had cheated on me but I was so insecure and in love with her I couldn't leave her. The strange part was that she admitted herself unprompted from me. I knew girls were more mature mentally than boys, but I never broke up with her.
Then I moved to another town for school and we were apart for 1 year. Deep down I knew she still slept around but I never wanted to leave her. She followed me after 1 year.
Our relationship had changed so much that I almost couldn't recognize the girl I fell in love with 3 years ago.
Then I moved to another country for 3 years but we still dated when I came home in the summers. My feeling had changed because I grew more in those 3 years.
After high school, when I came back home, the relationship was all but over but out of habit I wanted to get back to her but she had grown distant and looked like she had no feelings for me.
Deep down I was not surprised so I just stopped trying.

After 4 years away I came home and saw she had gotten one kid but still saw her in the social scene acting like she was a teenager and chasing all kinds of men although she had a kid and a husband.
One night after a dance, I accidentally met her and some guys were slowly following her and then she came up to me and grabbed my arm and turned towards the 2 men: He's my boyfriend. Clearly trying to get them to become jealous. They looked at me and kept going.
When they left I moved her arm away and started going home when she asked me: "Do you want to get back together?" I turned and looked at her: no feelings for her from our early years. All I saw was a another person. I replied: "No." and turned. Then she yelled: "Why? Do you think you are better than me?" Some people turned to look at us. I just kept walking away, when she ran up and kicked me on my leg while yelling: "You are worthless!"
I didn't react, just kept walking away.

People grow apart at different speeds. Just keep on going if you do and don't try to force something through because of feelings from years before.