I'm very sorry you have been having this difficult time. Each person is unique, but I do think many people suffer emotionally after breast cancer diagnosis and treatment. I've made a little list for you about what has helped me. Skip anything that doesn't appeal to you! These are just possible suggestions.
Actually writing this out was a good reminder for me about what I can do myself.
Wishing you the best.
***

Therapeutic:

Individual psychotherapy
Cancer support group led by a social worker on zoom or live (a dear friend participates in this).

Creativity:

Journaling
Drawing or any artistic pursuit
Taking a class in something that appeals to you
Reading in any area you like (a good novel takes me out of myself).

Exercise, PT, and Bodywork

All and any!
A soothing massage
PT for any lingering pain or symptoms of stiffness etc.
YMCAs have a program for people with breast cancer post treatment—gym, etc.

Diet

Consult with a dietitian (my sister found this very helpful, particularly to regulate her blood sugar during cancer treatment)

Spiritual

I like meditation and mindfulness practice.
If you already have a religious approach, lean into it.
Being in the natural world feels healing and uplifting.
Gratitude journal (I was dubious, but it really helped me).

Social

Figuring out who can be a support among friends and family (I gave up on some people and others came through in a wonderful way).
Mayo Connect—we want to know how you are doing!
Also—try making a list of things you enjoy doing and try to do them frequently. The possibilities are endless—but try rewarding yourself with fun as often as you can. Often enough it is difficult to just keep going. Give yourself credit!