I used to experience this a lot in my charismatic days. Since I've left those behind I've not had all the dramatic highs and lows.
I think often times it's easy to get caught up in things when you're in a group.
It reminds me of people in rehab. They spends weeks or months in a supportive environment, lots of people around to talk to or cheer you up. They're locked away from the rest of the world, away from people who will tear them down, push them to drink or do drugs, etc...
Many may feel confident than can maintain when they leave rehab, but once they're back in the real world, it's much harder.
The temptations are real, the pressure, the lack of support and encouragement, etc... can tear down any enthusiam they had, quickly.
I think it's similar for Christians in like environments. But under the guise that it's spiritual.
I will say that first getting saved is an exception. I remember first getting saved and I felt like I was seeing the world through new eyes. Feeling new things. Like a veil was lifted. But within a year it all had worn off.
And I think that's pretty normal.