RE: It's okay to not be okay (or: freeing yourself from the false "power of positivity")
Thank you for posting about this and sharing your experience.
I totally agree with you that it's ok not to feel good every time. Also trying to get rid of unpleasant thoughts and feelings make thoughts and feelings more believable. It's like telling yourself not to think of a pink elephant. You end up thinking about it more! Same goes for unpleasant thoughts.
I like Tara Brach and watched some of her talks on Youtube. I especially liked her RAIN exercise which you probably know about.
It's great that you are getting helped by MBCT. There is a similar type of therapy called Acceptance and Commitment Therapy that you might want to check out. I'm reading books about it and are using ACT concepts and techniques and helping me a lot. There is also Mindful Self Compassion that might also want to read about.
More power to you and have a good one!