"I could do it." It's possible that I have done it.
"I would do it." I intend to do it, but something is stopping me.
"I should do it." It is correct, proper, in the best interest, etc, that I do it.
I would make too many mistakes. This implies a contrast, like in the sentence above. "It's likely that I will make too many mistakes." "I wanted to have foreign buyers over, but I was worried I would make too many mistakes."