1 Trapping and rebreathing carbon dioxide would speed your breathing up, Tim, not slow it down. The breathing process is not regulated by oxygen, but by CO2 level. As CO2 level increases, breathing rate increases in order to get rid of it.  
On the other hand, oxygen level can decrease to the point of being lethal, and if the CO2 has not increased your breathing will not increase at all.  
That's one of several rather odd, sort of contra-survival characteristics of the human body. People are found dead of dehydration in the Grand Canyon, for instance, with full canteens of water on their belts, because dehydration does not make you feel thirsty.  
Normally, as your oxygen level diminishes your CO2 level will increase, because CO2 is the result of your body metabolizing oxygen. There are circumstances where that does not happen, and wearing a cloth mask is an example of interfering with natural process, which can lead to unnatural outcomes.

2 Tim, This was a neurosurgeon's thoughts on the subject, via Megan Fox at PJ Media:
https://pjmedia.com/news-and-politics/megan-fox/2020/05/14/neurosurgeon-says-face-masks-pose-serious-risk-to-healthy-people-n392431
My only thought was that, since so many asymptomatic people are running around with the virus, how does one know?

3 Bill's right, Tim. I wonder if your doc's tactic is just to try to calm you down with the mask somehow. Some psychological dodge. He's not trying to do away with you, because then he won't get paid.


Frankly, from what you've told me about everything that's wrong with you, I'd think that you'd either be prepared for anything, or nothing. But hey, that's just me. But I'd say to that sawbones "Get that damn thing off me or I'll crush you!"

I'm already tediously on record as to why I don't like wearing those damn things. . .

4 Well guys, I do know it slowed MY breathing down. It's why people who are hyperventilating breath into a paper bag, after all.

5 Bill here's a Web MD link explaining the use of a paper bag to slow down breathing. https://www.webmd.com/lung/lung-hyperventilation-what-to-do

From the article:

"You breathewithout thinking because your body does it for you automatically. But things can change your breathing pattern and make you feel short of breath, anxious, or ready to faint. When this happens, it’s called hyperventilation, or overbreathing.

That’s when you inhale much deeper and take much faster breaths than normal. This deep, quick breathing can change what’s in your blood. Normally, you breathe in oxygen and breathe out carbon dioxide. But when you hyperventilate, the carbon dioxide levels in your bloodstream drop too low. You’ll notice it right away because you’ll start to feel sick."

End quote.

While long term high carbon dioxide does make you breath more, if you are hyperventilating (and when you see a guy coming at your eyeball with a needle you tend to hyperventilate) a short slowdown of breathing is needed.

Again, as I say, good for eyeball shots but not so good for driving.

(BTW they don't recommend breathing into a paper bag anymore; blood carbon dioxide levels can jump too high.)