I have this and I always struggled with math. I was terrible at mental math and I wanted to be better. I changed all of my clocks to military time so I had to do a very small and easy mental math problem if I wanted to know the time. I started to get better at it and now I am better than most people I know (not by a whole lot, and it does still take me a while to get more difficult problems done). I now enjoy trying more difficult problems just for fun. I also developed some interesting methods to chunk problems in my head so they are easier to manage.

It’s not about hours of practice. Small incremental changes over time can be a better way to go about it.