Active users is the standard metric used to check how much a service is used (at least as far as i know. its what i see when i look at stuff published for investors).
hexbar is on the sixth place in term of number of active users with 1.8K , lemmy.world is 18K (enable the "active users" column and sort by it to see the full list)
It's pretty good for self improvement. I scored kinda high on neuroticism but after learning some psychology stuff like mindfulness/meditation/neuroplasticity/stoicism/ACT i managed to reduce it significantly after about 2.5 years. there was even a study recently that showed how using a app to change your personality and it's not easy and require serious training like learning a language or a musical instrument. I hope we will get eventually some open source version of that app.
and of course you need to be open to it, humans have a documented bias toward not believing what might hurt there self esteem.