Just say "No" to ________

I'd like to be able to learn how to do this whole "saying no" business and I thought I was actually on track for a while. I've been saying "no" to a conducting job since the summer but they've finally worn my down and I've agreed to fill in until their July 1st gig is over. I'm a moron and I know it. Please, someone slap me. Please... I need reinforcements here.

I agreed to be a mentor (laugh now) for someone's master's thesis which means I'm basically their editor and might as well be their supervisor because their supervisor is apparently content with whatever I say is good because the supervisor know nothing about music - his words, not mine. I spent 1.5 hours editing tonight instead of going to the gym like I wanted to do and now I'm starting to get anxious about all the work I need to do there and at my actual job.

Again... please, someone... anyone.... slap me.