I was excitinged during the classes., I was excited during the classes,

It would be better if you wrote "I was excited during class" but since you might be referring to specific first aid classes here I did not change it

Eespecially when I passed the exam. especially when I passed the exam.

Combine with previous sentence

I felt that I was powerful because my result was better than my expectation. I felt powerful because my result was better than my expectation.

Or "I had exceeded my expectations"

It was easier than myI had imageined. It was easier than I had imagined.

But I thought that maybe that was just because it was a very short term course. But I thought that maybe that was just because it was a very short term course.

I'm not sure I'll have the same energy to focus on awhen I complete the long term course. I'm not sure I'll have the same energy when I complete the long term course.

If you are definitely going to do a long term course in the future, you can keep "I'll" If you're considering a hypothetical situation, "I'd" would be better and you can keep the rest of the sentence as is