Grieving my father took 2 years or so to overcome. Grieving my brother took about 4 years. They were different situations and relationships, so I can’t ascribe the difference solely to therapy.
I will just say that for me, the only way to get the grief to go away is to feel it. Journaling, music, solo hikes, therapy, whatever it takes. The nice thing about therapy is that it holds you accountable. You have to think about it for an hour a week. Now, you can bullshit your way through therapy and stick to surface stuff, but then what’s the point. If you really crack open the shell and let the anger, despair, confusion pour out, it will eventually get better.
Either way (therapy or no), it is like a deep cut. There will be a scar. It will never go back to how it was before. But I hope you find some peace.