I wasn't quite that poor, and it was a long long time ago, and it still feels weird to look at my bank balance. A part of me still always fears that is will be taken away.
I broke into the middle class over 20 years ago, but I still have a constant nagging fear that this is all temporary and that life will bring me back at any moment.
I've done a lot to mitigate it though. My savings include a cash position that could get me (my family) by for over a year of unemployment if necessary. I have a good retirement savings. My son has a large college fund. I've also not been afraid to spend money on things that make me happy even if it's frivolous.
I don't feel like I don't belong in my position. It's just a worry that it could be fleeting, and I don't think that's ever going to be completely out of the back of my mind.