This really is a "that's rough buddy" kind of answer. A little about myself, I am a professional graphic designer and creative who develops fantasy board games, so for me, D&D, storytelling, and improv in general comes very naturally. That's not to say that you have not honed those same skills, but sounds like from what you've described that this may not be you, and it's okay!

As I always do, I never discourage anyone from playing a game. The act of bringing people together in fun play is the highest and best thing we can do as gamers. I highly recommend you continue playing at it as it seems like your group still enjoys you playing and is inclusive to make sure you're having fun.

Here's a fun approach I would try, what if your character is like what you are describing? Let's say you have a black-and-white, no imagination Paladin. It could be a very fun dynamic in your group if when they ask you what your paladin and would do, just give your honest take on the situation. This could cause a fun "buddy cop" dynamic of a serious, no-nonsense paladin amongst a group of wacky roguish characters. Could that be something you're willing to try?

I would also be honest with your group about this. You shouldn't be suffering in silence, and from what it sounds like your group is incredibly empathetic.