Trying to find a way to tell my coach my lease horse is too much for me

28F been riding most of my life but recently got back into it 9 months ago after several years off. An old coach I used to have had a gelding available for part lease which fit my budget and schedule perfectly. He used to be very fancy and advanced in dressage, though he does not compete anymore. Since coming back my confidence is obviously a bit rocky. This horse is LOVELY but has so many buttons that are far more advanced than I am, and I’m honestly a bit scared of cantering him. Walk/trot he is great, but he can get very spicy went transitioning into canter (which is likely due to some of my unbalanced seat from so much time off). I’ve been bucked off several times and came close to it several other times. Honestly my confidence in cantering with him is shot, and I think he’s just too much horse for me. Is this something I can overcome with him or should I be getting practice on a more solid/school horse type before I get really hurt?