I decided to leave my boyfriend and within a few weeks he was diagnosed with cancer

I have been with my boyfriend for 9 years. We have two kids. I found out within in the past several months he was unfaithful (4 times with proof with one girl over a one year time span) on top of how controlling and accusatory he has been to me since we had our son 4 years ago. I decided at the end of the year I'm ready to go be happy and find my life. I want to leave and give my life a shot. I'm 30 right now and have spent my entire 20s with him and dealing with mental and emotional abuse. We have not separated homes yet, but I do tend to sleep other places and we don't share a bed anymore and haven't since the separation. Recently, roughly 2 weeks ago he was diagnosed with stage 2 testicular cancer and now I'm fleeting with feelings. Am I supposed to stay and work with this so he isn't alone during what will probably be the worst time of his life? Do I continue to support him from afar? I've sobbed every single day.

Edited to add: yes, both my kids are his. His attitude changed after our son was born. You all have given me great advice and I am grateful. Something that stood out the most to me is therapy. I think it will be needed for me and my two children regardless of the eventual outcome. So I can work through feelings I don't understand while also showing my kids how to be supportive when people need help while also sticking up for yourself.