Should I let go or fight for our friendship?

So it's been almost 2 years since my ex girlfriend and I have broken up. We were young, our relationship started off fun but became very unhealthy towards the end. Eventually we ended in bad terms. I heard about her once in a while from mutual friends, I tried to move on and eventually I did, so did she. We became friends again after a year, and it was great. It didn't matter who we were both dating, we would still have the best conversations.. and there was actually "trust" there. But we were just friends.. normally she'd call me once in a while but as the months went by, we started losing contact again. I haven't heard from her for about a few months now.. I saw her recently, but we walked past each other like complete strangers.. and I have no idea why this is happening. Just when I thought I was over her, no matter what I do or who I date, I can't seem to stop thinking about her. And now I'm confused.. I feel like I'm going to have to get over her all over again. :( What do I do?