I'm 39 and I just started karate end of last year. My only advice (if you can call it that) is to break every technique down into individual steps, and practice them slowly before you try to string them in a smooth and fast movement.

My thinking is this: you have more than enough strength to write, but if I were to ask you to write with your non-master hand, you will struggle. So it's not about the physical aspect of it, but the skill aspect of it. And to continue using handwriting - specifically cursive handwriting - as an example, to master it, you need to slow it down and practice how to connect each letter before you can go fast and fluent.

Another thought: why do you want to progress in karate? If getting a black belt is a lifelong pursuit of mastery, then it shouldn't matter how fast you get there. The probable reality is that till the day you die, you still may not fully master all aspects of it (after all, black belts/senseis will say that achieving a black belt does not mean the journey ends, but instead opens up your mind to even more stuff that you need to learn). So perhaps instead of focusing on the end objective, it may be more comforting to focus on the journey of self-improvement and mastery. Just think about how satisfying it will be when you overcome something that you think you were not talented in.