nostalgia is a negative emotion for almost the exact reason you described
Nostalgia often is described as bitter-sweet, I agree it has negative tones to it.
The bitterness under the lens of nostalgia comes from wanting to be back in the past, which is a reason some call it distasteful, unfulfilling.
Nostalgia is a mixed emotion for sure, yet do you think there are people who find nostalgia more sweet than bitter? That's what I've found.
The reason I call it distasteful is under the lens of reflection. To look at the abandonment, trauma, attachment, etc with a clear eye, no fondness.
The ones you want to be in based on previous environments can still be.
Another important thing to note, yes, we build ladders from our base, or past experiences. Some can!
I just personally found that sometimes the base is so rotted from delusion and harmful beliefs that one must start fresh.
I.e. Matthew 7:24-29
This is not being free of nostalgia
Yes, to remedy yourself from the haunt of the past with a clear eye requires working through the discovered trauma and attachment, which is distinct from nostalgia.
What I wanted to note here is that such conclusions of the past aren't there with a nostalgic perspective. Nostalgia clouds the eye.
Those enamored with the past disregard their uneducated, unprepared, ignorant nature.
Everything in the past was "right" and it's the current world that is wrong.
To instead criticize the current reality instead of oneself. To idolize the simple past.
So you first have to free yourself of nostalgia, and then you'll have to free yourself of attachment and trauma, as you've said.
It just seems that one can only see such attachment and trauma when one sheds the fondness for the past, which is difficult depending on your mind-makeup.
You have to see the pitfalls. Reverting this transfiguration is difficult.
Some could see their ignorance as either their salvation (let's go back to the past) or bane of their existence (I need to fix myself and move on from such things).