Here is my personal experience with my own MC and this might be a little spoiler-y but I want to share it in case it helps anyone. Due to a lot of gaslighting and mental abuse my MC doesn't believe he is strong even though from the outside looking in we can see that he is strong physically and has the capability to be powerful and it is alluded to that he has supernatural abilities from the start.
He started out as a Gary Sue several years ago and I really had to dig in and figure out what exactly made him powerful though. I had to find what his weaknesses were, why they were there...etc. So basically, as the story progresses and he learns more about himself and heals he becomes more aware of his potential because he understands that he already is strong and powerful, he just wasn't able/allowed to see that.
So my advice is: find out what makes your character powerful and where it comes from. If it was given by chance, it is highly unlikely that they will just immediately be able to control it well. If they were chosen, then why were they chosen? What are their weaknesses?
In terms of my character, the mental battle within himself is his biggest struggle, not the physical fighting he has to do. One could view him as weak/the underdog due to his inability to just see that he has the power to change everything around him in an instant, but that's how scary mental manipulation can actually be.