I was feeling sadness and pain myself before I found this forum just an hour ago! I can't tell you what to do but I can share what I've been working on myself.
I have found the further I separate myself from my ex and the more I focus on the present and on myself the happier I am. When I think about him the grief, pain and feelings of loss are still there inside me, it's almost been a year since we separated. However the time I feel these painful feelings is shorter and shorter. I acknowledge the grief but I try not to feed it. I think it is very normal to feel the way I do, I love him, I miss the happy times, I have grief, regret, remorse, but I do not miss the mess that our relationship had for the most part become.
When I find I am thinking judgmentally about him or my past behavior I try to replace the thoughts with loving compassion, compassion for him and compassion for myself. I did my best, he did his best I remind myself. When I find that I am rehashing the past or missing him I ask myself "where am I now, what am I doing now, am I happy?", it re-centers me and the answer is usually "I am here, I am ______, yes, I am happy".
I am experiencing more of a happiness and connection to myself then I ever recall feeling. I am so happy now, not that I don't have my down times, but I am reconnected with a joy and contentment I never knew existed before. The surest way for me to forget my happiness and experience stress, insecurity and worry is to start thinking about my ex. It's a blast from the past! A past I do not want to bring into my present. Distract, distract! Those memories are like a hungry monster, the more I feed them the scarier it becomes, the unhappier I become. I could wallow in misery for days, but I don't want to.
We both experienced so much suffering, enough is enough! We did have some wonderful times though, beautiful times. The difficulty doesn't invalidate the delight, I am grateful for it all. The greater self awareness and growth I have experienced as a result is precious.
I know now without doubt (though I sometimes forget) that life is a blessing and a gift. I try to never forget the beauty each moment can hold. The end of a relationship is a loss, grief is understandable, but it can also be an opportunity. I can give energy to my suffering or give energy to my full acceptance of what is. In each moment I have choice and an opportunity. When I focus on what is instead of what was or what might be I am present and alive. Things change, feelings change, I try to remember this. Pain is not forever, joy is not forever. All of this is part of life and when I am focused on myself and in the moment I am able to be an active participant in creating a life I can enjoy instead of a victim of my unconscious fears and negative coping mechanisms.
Best wishes!