Backpacker insurance is an insurance category of its own. I've seen a few content with trips up to 24 months. Some allow multiple trips back to base (almost like annual insurance), and others for a single trip, just longer.
When I first looked - first time I was travelling for longer than 3 months, I was astonished to find these sort of policies available to those up to 65 years old. Some others just go up to 25/26-ish. Places like moneysupermarket, comparethemarket or gocomparre are a decent starting point.
As
@azure says, make sure your diabetes status is clear, and they're likely to want additional information from you.