Congrats on 12 years, that shows that there is love & affection between the two of you, & you can get through this. Communication is key - you need to talk to your husband about what you need & enjoy in bed. Make sure to do it in a way that doesn't put him down for his current shortcomings, letting him know what you do enjoy that he does, & asking him if he can do more/less of something else.
I went through this recently with my own husband (16 years this May), asking him to be more affectionate outside of the bedroom, & to offer compliments out loud. It took a while of encouragement from me, but things are going well! The key is to make sure you recognize when he IS making an effort - it's so easy to fault all the times they don't, but if right now there aren't any efforts, then every one offered is a victory in the right direction. Don't forget to listen to what he wants/needs as well.
Regarding his sleep apnea, I haven't gotten my husband to the sleep clinic yet, but I did record him & play it back - he laughed nervously, had NO idea that he was snoring so loudly. (I don't know what else he thought I meant when I said I could hear him down the hall even with the door closed). I know he has sleep apnea, & allergies, & all sorts of other issues. But it's scary for him to think of having something so serious, & I realize going to the doctor & having it diagnosed makes it "real".
In the meantime, I spoke with a clinician @ a sleep clinic, & they suggested having him sleep with elevated pillows. There is a "wedge pillow" that you can buy, but for now, propping up on 3 pillows is working... I'm getting sleep! So, have a heart to heart with your hubby - let him know that you are worried for him, and ask him why he isn't wearing the mask. LISTEN to his answers - whether you agree with them or not, they are his feelings & they are valid. From there, you can work through how to make it work for him, & then for you. In the meantime, give him some extra pillows :)
Hope this helps. All the advice below on counseling - follow it. Even if it means talking to a more seasoned married couple for advice (either together or separately with each of them), we have found that to be helpful as well. Good luck! T. :)