Moms usually don't need as much affirmations during active labor before things get crazy intense at transition, during that time most want quiet and maybe some light physical support. At transition we start feeling and saying we can't do it and such a lot of the time, and reaching out for someone to help.
Some things that I've heard and also good affirmations I've read:
You can do anything for one minute (this reminds mom take contractions one at a time).
You're doing it!
You're doing a great job.
Birth is normal.
This is possible.
Let go. (really, a lot of the time giving in is what it takes here)
You're going to meet your baby soon!
A lot of the short encouraging stuff is great for mom to say as a mantra too with each out breath, like "I can", "open", or just "yes".
Supporting mom leaning in a standing position
Supporting in a squat
Smoothing tension in forehead, shoulders, anywhere
Hard counterpressure on lower back with a palm or fist or massage tool
Foot or hand rubs
Modeling calm, deep, abdominal breathing near her face for her to match
Giving drinks or snacks