1. Maybe with smoker lungs, but I have used a mask at work for 2 months straigth with absolutely no issues with breathing, maybe first day was a bit weird, but you get used to it. Consider getting a proper mask if you seriously have breathing problems, there is plenty of variation on the quality.
2. do you have any source that this happens, or is it anyway relevant to your health comperad to regular city pollution?
3. Your own fumes are hardly bad for yourself. And of course the fumes that come from outside, if they stop at the mask, it's just a benefit, dont you think?
And yes the mask probably doesnt protect the wearer, but it protects others from the wearer. Now if everybody would wear a mask we would probably be much better off with the infection amounts as the people that carry the virus without symptoms would not spread the virus, even if they never knew that they had it.
Of course the best course of action is to stay at home, so you dont have to use the mask all the time, But please put it on when going to shop for food or anything else.