I'm so sorry for your loss, I understand exactly what you're going through 
I lost my younger brother to cancer 5 years ago, he was 28.
The grief of watching him come to terms with his diagnosis and deciding to stop treatment was almost insurmountable. I knew how scared he was, and that hurt more knowing there was nothing we could do about it. The grief was one of absolute helplessness, raw and totally visceral.
The grief after he passed was for me, my parents, my sister, his wife and his friends.
It took a very long time to come to terms with how he passed, I met with a counsellor during his illness to help process it especially because of the hopelessness of the situation. All I could think was that I would try and deal with it as he did, with a wit, humor and determination that saw him through to the end.
Grief is a painfully slow process - even now I am crying as I type this as I remember it all too well. The only way I got through it was to talk about it, with friends, family and on here. The more I talked, the better I could verbalise my feelings and realize others were feeling the same way.
I'm so sorry that you're feeling these things, it's such a head fuck to process. My therapist told me to let all else go and to treat myself like I was wounded (which I was emotionally). Each time I felt lost and depressed, I was to find a small joy, basically any small thing that would help lift my mood, like a square of chocolate, a licorice tea or a crappy magazine. And to allow tears anytime I needed. I visualized the tears as a slice of grief leaving me so the more I allowed myself to cry the better. It's how we heal.
I don't know if any of those things will help but I hope they do 