How can I trust myself?

My ex classmate spoke shit about me during a job interview at the company I’m currently working at as an intern. She said I need to be told what to do constantly and she won’t accept an equal position to tell me what to do.

I have never spoken bad about her. On the contrary, I spoke well about her even if I dislike her because I didn’t want to ruin her chances of finding a job (I know how tough it is after graduation).

In what she is right is that I have a deep insecurity. I have to fight constantly against that fear that tells me everyday that I’m stupid, that I’m a retard. It’s been like that for a while, for many years to be precise, before I even noticed. I think I’ve felt stupid ever since I’ve been a kid. I feel less, I doubt everything I do. I lack direction. At work I’ve done stuff on my own more proactively, at some points I do ask what to do. Even the guy I’m dating said (in a different context), that I never decide on my own, that I should be more empowered.