I have a fixed bar at the back of my teeth on the bottom row. I'm 21. Thing is, I had to wait a week in between having braces off and the retainers made, so while my top set of teeth remained straight and I then had to wear a normal plastic retainer for just a year, the bottom row got a a small bit crooked in that week, and my teeth are now just slightly overlapping at the top of most of the teeth, but overall they are still far better than they were before braces.
I was told I'd have the bar until I was 21 or until it fell off, but then surely the teeth would move more due to the sudden lack of support? For my top row (which is still very straight) I had to decrease retention gradually ie every other night. I'm not going to risk bad teeth by taking it off, tbh, cos my bottom row was terrible before, and now it's actually not bad, but just not perfect anymore, and only the top row is visible anyway, and the dentist's concensus is to leave it in indefinitely.
With you, it is the top row so could be due to the overbite, but you never know. Don't worry, better for it to be there than not!
When I graduate and start earning I'm gonna eventually try and have a removable retainer made for my bottom row so I can start reducing the time I spend in retention, but I'm not that fussed about it at the mo', it's just annoying with flossing and getting food caught in it, and it would cost a lot anyway cos I'd have to have it done privately and made within a day or so so my teeth don't move more.
So the moral? Don't worry. People can't see it and it's not that big a deal. Straight teeth are the most important thing. If they move bacteria could trap between them and then you may get rotting teeth.
Also, can you not get the jaw operation still?