Im in a relationship and still miss my ex

I dated my ex for 6 years more on than off, towards the end of the relationship we had some rough times, but we always managed to try and fix things and work things out. It has been nearly a year and a half since we broke up. She had a boyfriend, whom she broke up with (get to that later), I am in a relationship at the moment and I have been with this girl for the past year, she's younger than I by a couple of years, not common for me, but she makes mw happy i guess. There are some things i would change but no ones perfect right? My ex ended up breaking up with her boyfriend who she dated for a year because of me, she ended up coming to my house without notice knocked on the door and asked to talk. She spilt her feelings out for me, and asked for me to take her back. Since I was in a relationship I had to say no. She cried in the middle of the street and nearly brought me to tears, I had to let her go, and help her to her car and say goodbye. A few months later now, i have dreams about her still, my Facebook posts from years ago pop up on from the “on this day” app showing the good times we had. And it brings a heavy feeling to my chest, and makes me depresses. We were each others first loves, my highschool sweetheart, we traveled the world together. We cheated on each other but we were just stupid teenagers learning how to deal with things. I'm not proud of what I have done as she is remorseful for what she has done. But now I face a tough decision, should I follow my heart, or do I let the past be in the past and start over new? This has been on my mind for two months now, and its starting to effect my health and my current relationship. Who has some ideas on what to do?