We definitely have to be more careful as we age. I screwed my back up last year (again) taking the motor (25 hp) off my boat. It wasn't the weight, but sudden jolt as it tipped over and I went to stop it. Had an MRI and it showed 3 herniated disks, a small tear and pressure on some of the nerve roots. (I do have an ongoing back issue) It's hard to let go and it sounds like you have the same approach. I had the neighbour's 16 year old come over and put it on this spring, he lifted it like it was a 6 hp. LOL I promised myself I would start saying I can't help lift something, help someone move, rush at any job, etc.
I hope you can recover quickly so it doesn't keep you off the water!