My guess is that your grandmother doesn’t accept talking about your diagnosis because she loves you and doesn’t want to think anything is wrong with you. You’ve said she is stubborn and opinionated. Yet you continue to argue with her. Why? Why is it so important to convince her that your are ill? Why not focus on the good relationship you apparently have with her and enjoy your time together? With all you’ve been through, you need comfortable time with someone who loves you a whole lot more than you need to win this argument.
Many people do recover from depression and PTSD. It takes time and attention but it can be done. It’s not about “forcing yourself.” It’s about taking care of yourself. Work on your mental health issues with your therapist. Read what you can about your diagnoses and figure out ways to support your own healing. Eat right, exercise, and find things to do that give you joy. Start and end each day by counting your blessings. It will help. Really.
I wish you well.