Two suggestions before considering a "bull"...
Hi Ms. Carol,
Diabetes per se should not affect your husband's ability to impregnate you if he can get it up, fuck you and ejaculate. Diabetes increases the probability of a man developing erectile dysfunction earlier (perhaps considerably earlier) than he might otherwise, but four years isn't very long. Thus, I suggest your husband have his sperm count measured then consult with an M.D. re. the results, if he has not already done so.
You, of course, are the other half of the "getting pregnant problem." Sexually transmitted diseases (STDs) are a common cause... perhaps the most common cause... of women being unable to get pregnant, and of those, chlamydia is most common by far. (As noted when you google it, it's "hard to spell but easy to catch.") Despite your precautions, it's conceivable you may have picked up chlamydia without realizing it — after which it may have run its course and left scar tissue blocking your fallopian tubes — also without you being aware of it. If so, that could have left you unable to become pregnant... but fortunately, your fallopian tubes can be "repaired," if necessary, with a relatively simple surgery.
Thus, if your husband's sperm count is adequate, I suggest seeing a woman M.D. for an evaluation of whether you have an STD-induced inability to become pregnant... or, if not, whether there is some other identifiable reason.
Good luck!
—Custer