Constantly thinking about my ex-girlfriend who's my best friend

To start off, I considered this girl my best friend and the one person in the world I cared about the most before we went out. I have told her this and told her that before we started dating. The reason we started dating and crossed that line of best friendship is cause she confessed to me saying she liked me. I responded saying that i'd give the relationship a chance because of who she is to me. Eventually I began falling for her in that romantic way. Our relationship ended short though, after 2 months, she broke it up saying that what she felt was just really deep caring like it was for me with her in the beginning.

Since then, she has had another boyfriend, but that boyfriend went somewhere far and they broke up their relationship because of that. I have a feeling that she still constantly thinks about him though and is waiting for him to return. I have confessed to her that i do still have feelings for her, multiple times when i am really down, but it never ends well and ends with us not talking for awhile. For this reason, I try to hide my feelings until it starts eating me up inside.

Recently too, I told her what i told her before our relationship, that I care about her more than anyone else, she responded by saying that she can't care about me in the same way I do for her. She then avoided me for about a month before coming to me and apologizing to me for what she did. At first I did not know what to do, because I was still mad at her for lying to me (telling me she's busy so we can't hang out), and avoiding me. But those angry feelings eventually went away and i'm back to square one, constantly thinking about her. We are back to our friendship but it's not the same as before our relationship/during our relationship and I still can't stop thinking about her and I always want to go back to what we once were.
It's been about half a year since our relationship ended.