Any advice you get from mentors at your university is likely to be more relevant than you can get from the physics forums. I know you state you are having a hard time finding them, by mentor, I expect you mean "advisor" and not necessary a person supplying monetary support like and internship. I expect if you are just looking for an advisor, you may have had one assigned to you when you entered university. You can start there. If you do not like the one assigned, talk to your friends who their mentor/advisor is, and whether he or she is easy to talk to. Maybe they will agree to be your mentor/advisor.
I believe it is important not to be within yourself for self study for example, unless it literally is a last resort. Eventually, you may need contacts to fill out recommendations, whether it be for graduate study or employment.
In my case, I did not cultivate contacts during summer and preferred self study. Looking back this was a mistake. I examined my coursework the previous year and shored up deficiencies. This was very good as far as it goes. However, if I cultivated a mentor advisor as a faculty member, how much better a recommendation they could write (Two cases)
1. He obtained poor marks in quantum mechanics, but developed an interest in the subject late in the semester and continued studying the subject throughout the summer. My review of the problems he came in with, indicate he resolved his difficiencies in large measure.
rather than
2. He obtained poor marks,but he wants to do graduate study. Why not give him a chance.
No person is an island. You are going to need people. I learned late, you can learn early.
If your grades are good and you are confident, you can study for courses you take as a junior. Still you are better off letting a faculty member know, perhaps they can help, or may even agree to take you on for a summer session, at later.
I know one summer I kept my eyes open and a few of us got together, and asked a professor to give a summer course. He did
Good Luck in your studies.
