Even though I'm quite far from the BMO, I feel exactly the same with competition maths, it seems like each year I do worse and worse on the UKMT challenges (I didn't even qualify for an olympiad this year). This is most likely a mental thing, though. I saw a post saying the difficulty here is to get out of the "death spiral" (I'm miserable at this -> don't practice -> get even more miserable -> practice less). Doing well at math competitions is something that requires lots and lots of training and practice, the top scorers take private tutoring/lessons early on, study olympiad maths books etc., and the boundaries for the UKMT as you move towards the IMO are catered less towards 'encouraging people to enjoy maths and problem solving' and more towards people who have trained constantly since they were young who will be the future UK IMO team. I've realised that I don't train for these UKMT challenges (rather I did them for fun since I enjoyed them) so of course I could improve way more if I did.