I'm not sure I'm going to make it. Do you think you can help?
What are you not sure about?
I am very nervous about this.
If you're worried about some event that is coming up, that is nothing to be ashamed of.  Just remember that you are prepared for this, and you shouldn't give up on yourself.  But if you don't think you can make it in your normal life, you should get some help.
I think I can do it. I can do this.
It's okay to be confident. You should make sure you balance your confidence with also be self-critical and working on the things that you need to improve.