You’re only a nice guy because you’ve learned to be nice, safe, and “small” in childhood.
It was the only way to be “accepted” and survive in an environment that simply didn’t understand who you really were.
The problem is that you still use the same strategies (aka choosing comfort, hiding your truth, not expressing your boundaries, etc…) even though your life has changed.
You're not the little kid anymore, but your subconscious isn't aware of this yet.
That's why you still act like a nice guy, though you know, consciously, you shouldn't.
It's almost as if a different "you" is taking over when you get triggered, and the people pleasing, lack of boundary, avoiding taking risks, etc... is inevitable for you to act on.
I think at this point, you already know all about the nice guy syndrome.
You probably read many books and had “aha” moments, realising you’re not alone.