I self-inflicted some pain like that for me as well.

I got close to the end of Act 2 and my friend said that the difficulty was too easy and I was breezing pass everything and to switch it to Expert. Well guess what. I didn't level, upgrade or put my Pictos in a way to deal with it. So most of my boss fights turned from a close-win into 45 minutes of me dying a lot. Like, I'd be one-shotted so often.

I then took some time and then cashed in some Lumina for stories and then actually put some strats in and went through the skill tree. I think it was worthwhile in the end but it totally changed the experience for me.