yes I chased an ex for a long time as well, although not with sex
I thought that we were so right for each other and all I could do was carry on being mates and she would realise that she had made a big mistake in getting rid of me
In the end she got another boyfriend, and one day she told me she was really in love with him which hurt a lot (although obviously I had people telling me "if you love her you should be happy for her", get real)
One time I saw her around uni and she just looked so happy and full of life, I realised how she genuinely did love her bf and he was having a positive effect on her that I never had. Suppose that was the point where it dawned on me that I was never going to get her back.
Mates said "at last, you should have been at this stage ages ago" but it didn't directly change anything I suppose other than leaving me with an empty feeling of hopelessness inside. I have met another girl who I am pretty good mates with who I like now, and who knows whether something will develop from it but still I think about my ex.
But then also I think of another girl who I was seeing and ended up finishing with because there wasn't really "enough" there from my point of view if I was honest. She still liked (and possibly still likes) me and tries to get in touch with me sometimes to hang out. I can understand how she feels and I wonder if she is trying to do the same, thinking that if she hangs around I will change my mind about her, and the harsh truth is no it won't.
It all reminds me of watching that German film "Das Boot" about a U boat in the 2nd World War. The Germans realise their submarine is stuck at the bottom of the sea and can't get out. One of the commanders says that all his life he had searched to find a place where no woman or mother could look after them and he could truly face the cruelty of reality - and now he had actually found that reality.
People will say things like "at least now you can move on" but that is not the reality. You can't control when you "move on", you just have to stare at harsh reality that something that you really want you can't have and there is zero prospect of ever getting it. In the long run you are almost certain to meet someone else and be glad that you didn't sacrifice yourself to the pursuit of someone who didn't love you, but for now you just have the cruel reality. But you aren't the only person who has to face that.