as I'd been off work for 3.5 months.
Sorry to be blunt, but I find that unreasonable.
Again, if you've suffered complications from diabetes (DKA, HONK, etc) then you have every right to take time off until you recover. However, taking several weeks off (much less several months) is unjustifiable. Just because you can (legally) take time off doesn't mean you SHOULD.
Again, this is relevant to this thread because I believe many of you are making a bigger deal about this then it actually is. Yes, it is a serious disease. Yes, it is life changing. Yes, it can be a lot to handle. However, nothing changed before and after the day you were diagnosed except that now you understand what is happening inside of your body.
By all means, people should take their time getting back to 100% and adjusting to the news. However, I fail to understand how you can justify being completely absent from your job for that much time simply due to a diagnosis. If you can go that much time without working, all that tells me is that you're replaceable in your job. Again, sorry to be blunt, but that is the truth.
Go back to work, take it easy, and learn to adjust to the new routine. However, don't make this a bigger deal than it is. There's no reason to make this a bigger deal than it is.