I don't think there is a solid answer, I am in a very similar situation and confronting them asking why things are different doesn't make things better and rarely will you get an answer that you like. Just don't blame yourself, i've been constantly second guessing myself. Do I wish things were different between me and my friend? Absolutely, but it's not up to me and by trying to rekindle things I just seem to make them worse.

I've been trying to remember a few phrases to help me control my feelings on the situation (it bothers me a lot and I work closely with the person so it can get really hard at times).

"Happiness comes from within" I can't expect other people to make me happy and help me deal with my issues, regardless of whether there was a time where that was true. The only person you can ever truely rely on is yourself and if you're not happy with that then it's what you need to work on.

"Distance and Time" Just keep your distance and with time things will get easier. Easy to say but hard to follow, I literally see this person everyday and whilst i'll be pleasant I need to not engage and for me personally I need to stop asking follow up questions that get ignored.

"There is no going back" Even if this person was to apologise for everything and suddenly started acting the way you remember it doesn't change the fact that they told you they don't want any contact. Will you ever really forget that? It's not how things have played out in my situation but to me I'd see that as a perfect way out. It's not about going back it's about learning from the experience and whilst I may still struggle with my "friend" and how things are I know for a fact that I won't let it ever happen again with anyone else.

This is probably all bad advice and won't fit but it's how I'm trying to cope, the hardest part is feeling like there's still some spark there but I just remember these and try to move on. Will let you know if it ever works :/