Hi OP ,
here is an idea .... that you may not have considered .
Whilst not talking from personal experience of Sleep Apnoea , you may be interested to know some very interesting research/info about a possible cause.
It has been found that lack of sufficient Vitamin D , might be a cause. Your mention of long term fatigue also raises that possibility. Again , generalised fatigue is a symptom of Vit D deficiency.
Have you had a blood test ... ? IF not , then you might find it advantageous to do so.
IF it turns out that you are low, then you can improve your levels by supplementation. To get to a blood level of around 120 -140 nmol/L , may be a very good idea for you. You can get a test at this Vit D NHS Lab . <a class="break-all" href="http://www.vitamindtest.org.uk/" rel="nofollow" target="_blank">www.vitamindtest.org.uk/</a> . Unfortunately they do charge £29 if the test is not doctor ordered. It is simply a blood spot on blotting paper , you send it by post , and the result is emailed back within a week.
Here is a podcast , from the Vitamin D council , which explains the mechanism of how Vit D might act.
<a class="break-all" href="https://www.vitamindcouncil.org/?powerpress_embed=453486-podcast&powerpress_player=mediaelement-audio" rel="nofollow" target="_blank">www.vitamindcouncil.org/?powerpress_embed=453486-podcast&powerpress_player=mediaelement-audio</a>
The discussion is in American Units ..... ie ng/ml . You must multiply the number by 2.5 to get to the UK units of nanomol/L ( nmol/L ) So a mention of 50 ng/ml in the american podcast represents the exact same blood level of 125 nmol/L in uk lab results.
I hope this of interest , and might prove helpful.
IF you want any more info ....then get back to me . I have posted quite a bit on Vitamin D , here on Mumsnet ... you might find it interesting to have a look at some of my posts about its effects when our levels are lower than optimal. Many people here in the UK are deficient.
here is just one of many threads. See if you can recognise yourself there. It is a long read , but lots of useful info.
www.mumsnet.com/Talk/general_health/2421492-Vitimin-D-can-it-have-this-effect
Best of luck ,
BTBH