Should I choose a research field that I'm not really good at, but have strong interest?
Doing research on a subject is the best way to get a good understanding of something.  Presumably, if you do reasonably well in the parts of your coursework covering algorithms (in the eyes of the professors), you're doing well enough.  Mathematics and rigour are hard, even for most people who are capable of doing good work in the area.  Also bear in mind, humans are subject to the impostor syndrome.  If you find a professor who knows you and is willing to guide you in research in this area, then I don't see a problem.