Years after a four year relationship I'm still in love

I had a four year long relationship with a woman, beautiful inside and out. The end of the relationship was brought on by how poorly I reacted to treatment of a mental disorder I was diagnosed with.
I slowly recovered as I was put on new medication which I've now been taking for years. I've been symptom free for about 4 and a half years now as long as I take my meds. I'm back to my old self you could say. About two years after our relationship ended she randomly came over and suggested we take a shower together like we used to. Naturally I said yes but in the middle of it nature called and I had to stop it, which I think she took to mean I didn't want to be close with her again. I didn't contact her because she made her wishes clear when we broke up that I was not to contact her.
Fast forward to now she posted photos of her and her new boyfriend on instagram and I'm really jealous and sad. I've never stopped thinking about her and I still love her.
How can I deal with this? This is the woman I asked to marry me and she accepted up until our breakup. How can I forget about four perfect years with a woman I'm still madly in love with who probably doesn't think of me anymore?