It may sometimes seem as though the grades are unfair but it can be just a hard marking scheme. For my professors, some of them have class test averages where a 60-63% is considered amazing, others where a 75-78% is considered average for that course. If the issues are still going, then try to tell the teacher about them, they may not grade you easier though, they may help you out. If not, then ask your friends, get a tutor or simply study online.

As silly as it may seem, youtube can be great as there are many very good professors and even teachers who post videos, and compared to previous professors or teachers, some of these youtube ones are amazing. You may not always get your question answered by them but generally the more mature users watch them so sometimes you can get an answer from them, other times the maker of the video is more active so they may respond.