I read about exactly this once in a book published by top psychologists....they said the most probable cause was rampant homo***uality...do this sound like the case? Ha ha, I couldn't resist a smart-ass response. In all likelihood, you're probably just over-thinking it. Sometimes the holidays throws people off. You'll either get back to your previous form, or you'll learn from this experience and get better from it. The last thing you want to do is beat yourself up over it. Every negative experience in boxing is a learning experience, remember that. Like I said, don't over-think it. Good luck!
Maybe the trainers at the gym aren't good at going beyond the beginners stage. I dunno just a thought. Maybe because he's more experience he's got you figured out more. This happens. I can recall guys I sparred with in the past that gave me trouble but I felt I got better against them with time. I can also recall a vice versa situation where I used to always outbox this one kid in sparring and then he took like 4 months off for wrestling. When he came back he started off slow but within a month he was giving me a tougher time that ever and was more agressive than ever. I don't know maybe he went on the juice while he was wrestling because he took time off from boxing and seemed to gain ground on me.