What to do when completing a full-time masters by research, and a part-time job interferes with grades and progress?
It is most certainly possible to get a research degree while working part-time --- or even full-time.  I know a number of people who have done it.  As you are finding, it is quite rigorous, however.  In many cases, it may be better to take the coursework part time, so that you have enough space to really absorb the material, rather than just trying to hit passing marks.