When you do not trust yourself it means that you don't believe in yourself and believe that you can do something that you're trying to do.
Like I would say I don't trust myself to install a ceiling fan or anything else to do with electricity so you basically just don't trust in doing something yourself.
Just like you wouldn't trust someone else to do something for you and it's the same thing except you're not trusting yourself instead.
It can be hard to start trusting in yourself to do things but if you work towards the goals and learn how to do something then you can trust yourself eventually to be able to do those things.