I'm so sorry for your loss.
Trust me, it wouldn't have been any easier if you were together over the past year. You probably would have had the horror of finding him dead. He would have gotten the xanax anyway--that part wasn't your fault. In fact, NONE of it was your fault. All the "I wish"s and "if only"s are just a product of your grief, not based on reality.
Please look into finding a grief support group or a counselor. These horrible feelings WILL pass, but they will pass more quickly if you have the best support as you work through it.
Hugs,