Swallowing your own saliva while it is still in the mouth does not break the fast, and the scholars are agreed on this because it is impossible to avoid and Allah does not burden a soul beyond its capacity. However, if phlegm or mucus comes up to the mouth, one should spit it out and not deliberately swallow it, since that is possible to avoid; most scholars hold the fast is still valid even if it is swallowed, but avoiding it is safer. This is all built on the principle that the religion is one of ease, as Allah says He intends ease for you and not hardship.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.