I've always had trouble breathing through my nose. I can do it, but its like breathing through a straw; I just cant get enough oxygen and after 30 seconds or so I'm gasping for breath

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Normally this doesn't bother me but its becoming a real pain in the bedroom, because I can't go down on my man for more than 30 seconds at a time without fear of passing out; what a buzz kill. The problem gets worse too if I try to take him all in, as he's big enough that I think it blocks the airways at the back of my throat.
So I've gotten used to supplementing oral with my hands, and usually end up jacking him off instead. But I know he loves it when I suck on him and I want to do it more! I'd love to be able to get him off with just my mouth.
Has anyone got any tips that might help make giving oral a little more comfortable for me?
Thanks in advance.