Right so this kind of account always gets me because the tree line detail is really significant - objects that hover just above or within tree height are reported way more often than people realise, and it barely gets discussed compared to the high altitude stuff.
Where in the country are you? I'm in Nottinghamshire and we've had a cluster of similar reports in the Sherwood/Ollerton area over the past few years, always low and slow, usually described as having no navigation lights or the wrong coloured ones.
The "can't stop thinking about it" feeling is something a lot of witnesses describe and I think thats actually worth paying attention to. Not in a woo way necessarily, just that your brain is trying to reconcile something that doesn't fit any category it already has. That's a genuine cognitive response to anomalous stimulus.
Did it make any sound? And roughly how long were you watching it before it moved or disappeared? Those two details narrow things down considerably in terms of ruling out conventional explanations.