My friend is going to a make-up party, but I will not go.
Why don't you want to go?
It's hard enough to do my own make-up, let alone someone else's.
I can understand not wanting to work on anyone else's make-up. Either way, if you don't want to go to a party you have no obligation to do so. Just stay home and relax.
No, I might let someone down if I don't go.
I understand feeling like that. You can tell her you will go, but you won't do the make-up part of it.