That feeling of not being able to stop thinking about it is so familiar to me, that kind of mental loop where you keep replaying it trying to find a rational explanation and coming up empty. I had something similar a few years back near a reservoir outside Cardiff, just this silent shape sitting above the treeline and then gone. No sound, no trail, nothing.
What really gets me is the silence. Every conventional aircraft makes noise. Even drones have that buzzing whine. Did you notice anything like that or was it completely quiet?
Also really important - what time was it and roughly what direction were you facing? Sometimes cross-referencing with other reports in the area can throw up patterns. There are a few tracking sites worth checking against. Would be interested to see if anyone else in your area has logged something similar recently.
Please write down everything you remember now before the details start fading, even stuff that seems trivial. The shape, the colour, whether it moved or held position. All of it matters more than people realise at the time.