Well, obviously the right person DID indeed believe you. Time to focus on that and move forward. Why are you thinking you are not believed, then?

Yes, in going from high school/secondary school of any stripe to university there is *always* a change in levels and almost always a change in grades. Even if the material itself is not too challenging, it's a totally different environment, structure, style etc. This is normal.

I'm a lot older than you are and I'm in post-secondary studies for the second time. I still had a lot of hurdles to overcome along the way. The first two semesters I got straight A's and I thought I was pretty hot stuff. Then the material got deeper and the grades got tougher.

Remember, A's and B's are above average. C work is average work. It's okay to get C's. It's okay to have to resit or get an extension when you are not doing well. I went through a godawful depression flare four semesters ago (this is my ninth semester; I'm going part time) and had to get special considerations from a couple of my professors. Life happens, we dealt with it and moved on.

You're going to be okay.