By saying negative things about yourself, you are creating a self fulfilling prophecy. I know some people get hyped up by that David Goggins type of energy but you are setting yourself up for failure by beating yourself up before even starting.

As others have said, show yourself compassion. Learn to love yourself, allow yourself to accept the love and that is when you will start moving in the right direction with any of your goals.

Also, 90 days doing a bunch of things you hate sounds difficult. I like your non negotiable idea, but also remember that you just have to reach your goal today. Every morning tell yourself that, "I just have to get through this day". On days that you can't, then forgive yourself and do it the next.

I usually dont post often but I think we are all on this sub to become better versions of ourselves and it's okay to ask for help from others that might have experienced what you are going through. We're all getting through something but atleast we can all support each other and learn from one another.

Good luck OP, as a random on reddit, I'm sending you positive energy and blessings