Hi Dana, unfortunately this isn't going to get any better until he finds the cause and does something about it. However, it sounds like he's too busy being offended to actually look at what may be causing it. I know you don't want to talk about it because of how he reacts, but you've given him ample time to address it and he's done nothing.
Causes of loss of libido are many and varied. Let's start with looking at what's normal for him. Has this been a problem in all his previous relationships? If not, what has changed since then? When you have had sex in the past, was he really into it...or was he just going through the motions?
As for what's going on now, the causes could be psychological or physiological. For example:
PSYCHOLOGICAL
performance anxiety
unsatisfied in the relationship
not attracted to your body type
stress
poor sleep
anxiety
depression
PHYSIOLOGICAL
side effect of meds
unfit
overweight
And there's a whole host of sexual issues related to diabetes [url=http://www.mydr.com.au/sexual-health/diabetes-and-sexual-health]Diabetes and sexual health - myDr.com.au[/url]
And these are just off the tip of my head. Next time you have this talk, tell him that you're not asking him to get into bed with you. Tell him that you're asking him to see a doctor and try to figure out the possible causes and cures.
If he refuses to see a doctor about it, you're really going to have to rethink the relationship. You may love him, but him refusing to address the problem only indicates his own selfishness and lack of awareness of your needs and desires. Unaddressed, this is only going to get worse - not better.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.