I'm questioning whether or not I want to be my friend's best man.
Is there some reason why you don't want to be his best man?
I don't want to have to give a speech at the reception.
I understand why you're nervous about giving a speech, but it's important to be there for your best friend. If he wants you to be his best man and give a speech, then you should do it. Maybe you could get some help writing the speech.
I don't know if I'll be able to do it. I'm so nervous.
It's okay to be nervous, but I don't think that's a reason to not be his best man. Your friend really wants you to be a part of his wedding, and to be there for him during this big time in his life. I think should do it anyway.