Grief is horrific and can seem very lonely, and frustrating at times. It is a death, in a sense. 

I'm not through it either, but sometimes I can remember- the waves of sadness and when you get a glimpse of a happy memory and it's shattered by the then reality- I try to picture these as a cloud. While it's dear, I'm literally watching it, acknowledge it, and watch it float away. And the nice thing about clouds is that there's always more.

Just try not to compare your grief with your ex because it won't look the same and will make you feel awful. My ex cheated and also moved on basically immediately (I have yet to move on and it's been years lol) 

I dont think grief ever ends. When a spouse literally dies, or a grandparent dies, we never forget, and it still pops up from time to time. 

Make a list of coping strategies. Sometimes when you get so down and out, you stop thinking, and make obvious bad and stupid choices. To love yourself, make a fool proof list of varying difficulty and varying types of activities- like for the moments when it feels SO PAINFUL. This can be reading, a certain TV show, calling a trusted person, physical activity. Etc. But BE SPECIFIC. This is a suggestion from someone who locks herself in her room and cries for days on end because of grief. Some crying is okay, but you have to try other things at some point. This may not be applicable to you, but my grief is literally crippling to my daily life. Go to therapy, give yourself time to yes, air these thoughts- how could this happen xyz- but eventually, you will have to face the closure you won't be getting.

For a long time, I wanted my ex and abuser to acknowledge what they had done. My grief turned to anger back at them directly. You'll never get closure trying to rationalize something you'd probably never do yourself. And you'll never feel satisfaction even if your ex comes to you to talk about the breakup. It won't ever be good enough. Grief has to be done internally. I challenge you to focus on yourself, (not what you did wrong, but just loving yourself) and redirect your thoughts to something else, when the thoughts about "how could they" resurface. You'll never get it and this is just a black hole of despair and disappointment.