You are not a machine and you have the right to happiness. I hope you find it.
I recommend therapy, if you haven’t tried it, and if it’s not working, I suggest different therapists, or different types of therapy. It’s okay to tell a therapist you’re not getting much out of their style/your relationship with them, and ask for their assessment of what you should be looking for. Then go look at that. Keep trying until you find something that clicks.
If it helps, in your shoes I would view it as a continuation of your parental duties. When she moves out, or maybe has kids of her own, you can continue the relationship with her and your grandchildren as your genuine, authentic, and - perhaps - happy self.