Telling someone, particularly someone you like, ‘No, I cannot do this for you right now,’ isn’t easy.
But it isn’t rude, either. At least, it doesn’t have to be.
Saying yes, when you really won’t be able to, when you would be breaking a prior commitment, or when you would just end up feeling frustrated later, that’s just wrong.
Tell them why, and, you’ll garner respect, not resentment.