You can remember the meaning of 自信 with this easy mnemonic:
I've always believed that it was important to have confidence in myself (自), and just recently, I found a letter (信) that I wrote to my future self when I was merely 10 years old, telling myself to be confident no matter what I was doing in life.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.