Entering a New Relationship With Feelings for Your Ex

Once again, I haven't gotten any emails as to any problems that I can help people with, so this is another personal general advice post.
This has come up with someone I am acquainted with. This person I know, I am told by a friend (so I have no idea what is actually true in this situation) that they have been texting their ex and telling their ex that the only reason why they are with the new person is because they are nice, that their feelings still lie with the ex.

I don't know if this is true because I heard it through the grapevine, but I don't really condone this act.

With that being said, I did the exact same thing a few months back (during the summer). I was heartbroken and I didn't know what to do, so when I was talking to this nice guy and then my best friend kept asking him if he liked me, then he wouldn't tell her, but he would tell me, so he told me did, so we started hanging out. The "relationship" (he refused to label us) lasted a month and three quarters (I am really good at lengths and dates and easily figuring these things out [my memory is really good]) and in that time my feelings for him never changed from a slight "I thought you were cute once in passing" feeling except I did kind of start to love him, as a brother (this is like the kiss of death of relationships), of course, I never told him any of this. Although I did try and get back into a relationship with him, but that was only because I was afraid to be alone after being in a relationship. We never got back together and I learned to be on my own for a couple of months before my current boyfriend asked me out. I had feelings for someone else the who time during that relationship too, so even before it started months ago (with the guy from the summer), it was doomed. It's not the same with my current relationship, I love my current boyfriend so unbelievably much.

To enter a relationship knowing fully well that you still have feelings for your ex is like assassinating your new relationship before it even begins. Everyone says that you can start to love your new partner and this is often true, but you will forever be comparing your new relationship and new partner with your old relationship and old partner, it'll never progress if you compare it to something.

You have to allow your feelings for your ex go away by spending time to be alone with yourself so you can truly find yourself before you get into another relationship. If you don't do this all of your relationships will be doomed, not to sound mean or foreboding.