Knowing yourself is a bit of a prerequisite in maintaining good friendships. On the other hand you will only really get to know yourself with the help and through the eyes of the true friends. It is thus a bit of a chicken and egg situation.
However there are two key resources I found helped me enormously and can be blindly passed on / recommended to anyone regardless of their current situation or mental state. I literally have only gotten positive feedback on having recommended these two:
In what kind of activities you are at your peak happiness. What characteristics are supporting these activities. The book ‘Authentic Happiness’ of Martin Seligman addresses this question, and I strongly encourage you to do the 240 question VIA Strength test. Then you can ask yourself whether you share these activities implicit from these character strengths with your friends ? https://ppc.sas.upenn.edu/resources/questionnaires-researchers/survey-character-strengths MEM�
The second know thyself test I found highly useful is the Gary Chapman text on 5 Love Languages, which again tells you how people can press your buttons on how to feel loved, and likewise teaches you the categories how you can express love in the way that your friends will register it effectively. It is of course targeted for romantic relationships, but the knowledge will aid you greatly in evaluating your communication in your friendship, as well as knowing how to be a good friend to your friends who might have different needs aka love languages to you.
Please feel free to recommend me further resources as these if you are convinced of them and think it is good to share them further.
Finally of course there is now the Friendship Personality Test