Is Willpower a Limited Resource?

The answer seems to be yes. So the Spoon Theory would apply here also.  You will need to plan how to husband your willpower to do your therapy instead of sitting on the couch. Hopefully your doctor knows about Spoon Theory and can apply it both physically and mentally. But I doubt that, you will need to figure this out yourself.
https://www.apa.org/helpcenter/willpower-limited-resource.pdf