Mucus in my throat causes me to become nauseated whenever i exercise. Can someone help?

[Kyle]

I like to play basketball, but there has been a problem that's been hindering me whenever i play. Whenever i play, i start to feel a gag feeling and i constantly have to step off the court to spit stuff out. I normally feel like there's stuff in my throat; i feel it right under the top of my sternum, where the clavicles are attached. Whenever i play, it feels like that stuff clogs up my respiratory system. In more detail, whenever i'm playing, this clogging also causes a gag/nausea feeling at that location. I start to salivate excessively, mucus from my nasal sinuses come down into my throat which i have to force into my mouth and spit out with the excess saliva. Overall in this whole process, i feel horrible. This is becoming a real problem because i have to constantly step out of the game and spit, and whilst playing, i can't perform well because of the nausea feeling. I've looked for answers from my coaches, doctors, and the like, but none of them can give me a thorough explanation and none of them can fix it. Does anyone have any idea what this is? Does anyone else have the same symptoms? does anyone know how to fix it?