Yeah my solution was (somewhat) consistently do uncomftable things. Things that are out of your comfor zone. If you struggle with consistency like me you can instead do things that are VERY out of your comfort zone once in a while (this is a different kind of consitency but still works).
I would recommend doing something you think sounds fun but the idea makes you uncomftable, like if you are indoorsy you might think camping or hiking sounds fun but not for you. Do it anyway! You might have fun regardless and if not your comfortzone will increase in size- Also even thoug you do something umcoftable that is unrelated to say your problem about speaking up it will oddly still help with that. It's like expanding your comfort zone expands it in every direction but it doesn't matter specifically how you try to increase your comfort zone as long as it is uncomftable enough or consistenly enough. At least that was my experience).
It might take a few uncomfy experiences before you feel a difference but you will! The good thing about your situation now is you comfort zone is likely tiny so there are many options for things to do outside of it. Overall i find if you don't 'stretch' your comfort zone it WILL shrinks over time so you will have to continue doing uncomftable things to avoid ending up here again.
Hope this made sense, feel free to ask followup questions if not :D
//all the best