I have recently assumed a new role/gotten promoted at work. My very first day of doing my new job, I had a half day followed by two full days off (scheduled before I knew about the promotion and couldn't cancel). Not to mention being behind three weeks on my old job (which I have to keep doing until it is filled as well and then train my replacement) from when I was training for my new one. I am quite backed up all around on my workload and 80% of what I do faces pretty tight and rigid deadlines.
This has meant having to face the challenge of reigning in my eager-to-please, often overly-helpful nature in order to get things done and do them well. That has meant having to tell people no. And even be firm about the fact that while I realize I used to help you by doing X-task, and so did our former coworker whose job I now do, it is in fact your
job and now that I don't have time to do it for you, you do, in fact, have to go back to doing it yourself.
I've had a much more polite, but very similar version of that conversation (referring to that last sentence) three times already and it's been less than a week. Twice it was with the same coworker. And yet I've barely flinched when I've done it.
The old me would have tied herself in knots before risking disappointing, upsetting, or annoying someone. If the old me did manage to stutter out that she might not get to their need right away, she'd have felt bad about it for a month. I much prefer the new me who realizes that her job and time are no less important than anyone else's. =)