I know what you're going through. First I tried too hard to just forget, it's no use and it just sucked all the energy I had from me. Being bipolar(?) doesn't help in that at all because with that going into depression means going really, REALLY down. But yeh, I was in your situation couple of weeks back, and it felt like neverending limbo. You can't forget, but you can try to focus into something different. That doesn't mean that you shouldn't deal with your feelings, but there's easier and more difficult ways to do it.
I personally just ignored my emotions, and dealt with the cold facts. After that, my wounds started to heal and in couple of weeks there remained only scars with occational pus.
Before I could do that I had to realize what part of me was really hurt. Oh no, I didn't feel sad because of some divine, universal law of love had been broken, it was my ego that was assaulted. You know, that little voice that whispers in your mind "me me me, mine, myself, ME!"? It's that part that wants to be loved, wants to get attention, wants to be valued by someone. Oh boy was it hurt when it lost those feelings it had got used to. Of course, every person is unique and my ego is especially jealous. It doesn't want to share one's love and attention being directed towards it with somebody else. So, what I actually felt wasn't sorrow losing someone, it was losing someone to somebody else. Getting replaced, that I couldn't stand.
After that, I metaphorically kicked those thoughts into some far corner of my mind and started to sort out my thougts to those which were actually real and to those which were just assumptions based on my feelings. Emotions aren't useless, but I think human can control them, or at leas aknowledge which emotions modern human actually needs. I don't need to suffer because of a person who doesn't give a flying **** about me. Exactly what you achieve by hurting yourself while that person who had nice times with you before is living happily his/her life on? Nothing, as long as you can't actually curse person with the power of your mind. My "love of a lifetime" moved on, and so did I. It was happy when it last, but people change with the time and nobody knows what we will become. I've focused on myself, doing things I like and getting to know myself, and now I'm actually getting along with myself. My relationship improved me by giving good memories and experiences among with those bad ones. Things after that are things I don't need to or want to care about, our ways separated and she's living her life and me mine. You are no lesser person after your partner leaves you! You just need to learn to love yourself again. Enjoy small things, try not to think too big picture and don't overanalyze.
My father once told me two rules that leads to happines: don't take anything personally and don't make assumptions. Pretty damn good advice, at least it helped me to heal and get back on my feet.
