The best you can do is to just learn stuff that is useful to you on a everyday basis. There's too much stuff to remember anyway, so there's no silver bullet for forgetting.
my take is this: if you forget, it's not very useful/valuable knowledge and you shouldn't feel bad about it.
For the mentors, you should join an Open Source project where people will die to have you contribute, and if you have questions and problems, you should ask. They're more than likely to help you, free of charge. But you must contribute and show the willingness to work hard.
Richard Stevens' UNIX Network Programming books: http://amzn.to/2lmH8hy The 1st volume at least is going through a simple TCP/IP telnet server and enhancing it with features as you go (single connection, multiple connection, forking, multi-threaded server), etc.
my take is this: if you forget, it's not very useful/valuable knowledge and you shouldn't feel bad about it.