Avira- from the language you use (and what I’ve learned- although haven’t opened that many non lubavitch hashkofa sefarim), here’s my hypothesis about mussar vs Chassidus:
With mussar you identify with your nefesh habehamis, and as such you must constantly humiliate yourself for your selfish motivations (selfish being the best of it). Nothing is pure etc. how that is conducive to true growth, you explain to me.
In Chassidus we approach the same makeup differently. We learn to treat the nefesh habehamis like a clown that no one takes seriously- but slowly identify more and more with the true self of a yid- the nefesh haElokis. As such, Yiddishkeit it’s always becoming ever more joyful, because you can connect on a deeper and deeper level with your true self, ie oneness with Hashem.