Paul,
I'm shaking my head as I type this my friend as I can't believe what I'm reading. I don't want to worry you but it sounds too close to home.....
My DH started having "odd" symptoms over chistmas, like you we found reasons for how he was feeling as it was christmas as it's a time when over indulgence is common.
DH started by having extreme reactions to food - DH has a very sweet tooth, whenever he had sugary food he would have an extreme high then an extreme low, for example we rarely have fizzy drinks in the house, especially full sugar ones but my Mum does! He drank a can of Coke and could hardly move on the sofa. The first week of january he started drinking 25 glasses of water a day, and peeing a LOT. He was extremely tired particularly in the evening, luckily DH was on gardeners leave at the time so it didn't impact his work/career. He was losing a lot of weight and had blurred vision too.
A simple urine test at the GP's and our GP said he was "probably" diabetic but he was to have some fasting blood tests. GP asked him if anybody was Diabetic in his family as he didn't fit the age/weight/lifestyle catagories to develop Type II. After speaking with his Dad is was revealed their are four other males in hid paternal family that are type I.
In February DH developed an uncommon but POTENTIALLY very dangerous side affect to Diabetes called Diabetic ketoacidosis he started losing an incedible amount of weight (half a pound a day) as his body's metabolism was all over the place.
DH is officially diagnosed as Type II diabetic but his pancreas is packing up and will probably become insulin dependent in the future.
I am not saying this is what you have.
I think you need to get to the GP's.
It may by something you can stop in it's tracks.
Good luck.