I've dealt with nasal congestion all my life. I have a narrow nasal passage and am prone to congestion. It's tough to train when you can only breathe out of one nostril. Waa Waa Waa!
I've tried a several different solutions, and finally found one that works for me: Saline nasal spray! I use it in the shower every morning, and although it's not a pleasant experience, it clears out my nasal passage and allows me to breathe freely. My training sessions have been noticeably better since.
I thought I'd pass on the advice, in case others struggle with the same problem.