What is a good way to overcome fear of power on stalls?

One thing that may be happening is attempting to "chase the ball", rather than looking outside at the actual yaw across the horizon. You can see it, you can use the rudder to stop it. When you don't do this, and especially when chasing the ball, sudden wing drops are common. If the ball is "outside" the lines and you are using rudder it to correct based on what you see with this instrument, the technique being used is not working, because it's not keeping you from getting uncoordinated in the first place. It's only telling you that you are already screwed, so to speak. When coordination is better, the wing drops are less to non-existent and a bit of buffeting tells you to pitch down, stall is over. The coordination aspect is often the most lacking as far as instruction/teaching, with lots of methods given that don't necessarily help students fly the airplane or result in repeatable reliable results.

It would help though to know what is the most frightening aspect of these for you?

Do them at a reasonable altitude..?

I guess that there is a lot going on. Keeping coordination with rudder and light aileron control, and the thought of spinning if it goes wrong.

Maybe consider getting some spin recovery training? Once you realize recovery in a light single, from a spin entry (or even a fully stabilized spin) is no big deal...power on stalls will likely be a breeze. You’ll feel much more confidence in your basic aircraft control too.

I agree. When I did my spin training in a 172 as soon as you let out any of the spin control inputs it would pop out of the spin right away. If you just let go of everything it would recover on its own. This won't necessarily work for all planes or even a 172 in all conditions.

We had so much difficulty getting our 172's to spin... It almost always ended up in a very steep spiral

Same problem. Needed to have the power on, wouldn't spin below 2000 RPM or so. Very FWD CG though.

Another thing I would do with students to work on rudder control was to have them stall the plane and keep the elevator full back keeping it stalled and using the rudder to keep the nose straight. The nose would drop at the break, then pop up and break again, and keep repeating this until you recover.

With more avionics in the back, this has changed on newer 172s.