I feel like my mathematical competence is fading Background:
I studied Physics and Mathematics (double major) in my Bachelor, and now I'm working in the field of (theoretical) biophysics (I'm a Master's student). I have had a fair amount of research experience so far which gave me a lot of ideas about how research is being done. Most of my contributions to such a project involved wiring a bunch of code, improving an experiment, improving the data collection process etc. but nothing mathematical.
Problem:
I have no problem reading papers, especially theoretical ones; I would describe myself as familiar with a lot of fundamental ideas employed in research. If I want to dig deep into theoretical a paper, I don't have a problem. But the issue is that I feel like my mathematical competence is eroding, both high-level and low-level. I do a lot of errors in algebraic calculations, forget how to integrate certain functions, etc. and I am internally scared from finding out I am bad at mathematics, given that I used to love mathematics.
But unfortunately, right now there is nothing that challenges and pushes my limits. Because, to my experience, physics students only use certain mathematical methods, so once you learn how to do those (all of which can be done via Mathematica), there is nothing that can push your limits, unless you are doing very mathematical research.
So, now whenever I need to calculate something, I go and put it into Mathematica because I have done the same type of calculations over and over again many times, and I don't want to do it again.
I'm afraid that if I start working on a theoretical project which requires mathematical competence, I'll fail because I haven't been practising the art of doing mathematics in a true sense for a while.
Question:
Given all these, how to keep up my game? How to push my mathematical skills even when I am not working on a theoretical project?
First there’s no magical “being good at mathematics,” so it’s not something you can suddenly lose.  If you want to get better you can practice more and get better, and if you practice less you’ll get rustier, but neither state is permanent.  Your anxiety around your math skills may be due to this kind of black-and-white thinking about math skill.
In terms of practical advice, one thing that really helps keep you sharp on the basics is teaching.  Maybe try signing up to do a little tutoring, or do something with the undergrad math club.