As you have already found, our brains are very good and quick at making associations. The associations that your brain has made in this case are: mask = insomnia, and, no mask = relaxation and sleep.
Your job now is to get your brain to make a new association which will negate the old associations. This new association should be; mask = relaxation and sleep. Now troubleshoot...what can you do to make this new association happen? If this were me, I think I would handle it this way: I would put on my mask while still up and awake (but nearing bedtime) and read a book (something I find very pleasant). I would read until my eyes began to get tired and I started to feel sleepy. Then I would have a "chat" (silent thinking) with myself: "Wow...I'm feeling really sleepy with my mask on...I don't want to fall asleep in this chair/sofa/recliner...I think I'll go to bed now and hook up (with my machine)". Once in bed and hooked up, with my machine on I would continue the "chat". "My mask is on and my machine is going and I'm feeling very sleepy. It's safe for me to go to sleep now. I'm so thankful for this mask and machine for how it makes me feel so rested in the morning. I will always sleep with my mask on...this is my lifeline...what a blessing. CPAP is my best friend ever.....I'm feeling really relaxed now...It's OK to go to sleep this way...I like this." I would keep the conversation going until sleep sweeps me under and my very last conscious thoughts are something, anything, positive about my CPAP treatment.
If I found myself wide awake after this restful "talk", then, without judgement in any way, I would just get up out of bed and out of the bedroom (with mask in hand), go get a cup of soothing tea and linger over it, put the mask back on and grab my book once again. The book must not be a wild adventure story...just something fun and relaxing. Then, after putting my mask back on, I would read until my eyes got heavy again. Then back to bed. Repeat this as many times as you need to until you finally fall asleep with your mask on and your machine running. Your desire/need for sleep will overcome you...with your mask on.
Repeat this process each night until your brain transitions with the new association: mask = sleep, comfort, relaxation, sweetness, a good refreshing night, etc.
Never, ever, fall asleep without your mask on and your machine running. It isn't safe for you to do that anymore. Tell yourself that over and over again. Believe it...because it's true.
Our brains are amazing tools and are quite malleable. You may be surprised at how fast your brain picks up again on this way of sleeping. The keys in working with it are patience and persistence. You must outlast your brain. You can do this!