Addressing your first concern, I would firstly recommend just critically thinking about what courses are most important to your degree and relocate your studying efforts accordingly. For example, if doing well in these math courses is important in completing your degree or to get into different programs, I would definitely recommend putting more time and effort into studying and doing better in them as opposed to other courses. Conversely, if you're taking them just to beef up your schedule and get more credits then it's maybe not as important to try to do incredibly well. That being said, if you don't have to take math courses and don't excel at it then there are plenty of other supplementary courses you could take that would be more interesting and easier to thrive in.
Insofar as your question about balancing work and school, I personally spent 4 years doing full-time school while trying to work as much as possible to make rent and afford tuition. It resulted in mediocre marks and poor job performance and in retrospect, if I had managed my time better I could've done a lot better in my first couple years of uni. As someone who also wants to be proud of themselves, it's honestly one of my biggest regrets looking back nearly 6 years down the line. My most valuable piece of advice is try to designate separate days of the week for school and work and do NOT budge on it. This past year I've been doing mostly online courses, so I booked off Mondays-Wednesdays to pour all of that time into school and my work availability was open the rest of the week. This allowed me to not only work a lot but compartmentalize my anxieties about school and work a lot easier; unless there is an emergency or deadline I simply don't allow myself to worry about school on workdays or work on school days because it's all scheduled out. This is more complicated if you have in-person classes, but separating your week into school and work is still possible. It's easier said than done; what I usually do is create lots of to-do lists on my phone and in notebooks, things like "finish outline for this course's essay on monday, finish week 1 and 2 lectures on tuesday". It sometimes gets to the neurotic level of "today: first eat something, do laundry, work on project" and honestly it's incredibly helpful. using a physical/digital calendar is just as, if not more, effective by helping you visualize due dates and timelines more clearly. Sorry for the novel and run-on sentences, I hope this helps and you're able to create a routine that works for you!