I'm a singer. i suffer from allergies and sinusitis. how can i deal with my conditions?

Treat congestion: Singers attuned to their voice are more aware & less tolerant of hoarseness, a symptom of nasal allergy. Breathing through the mouth, the result of nasal stuffiness, is the cause. The mouth can't warm & moisten the air like nose does so air reaching the vocal cords is colder & dryer. Oral antihistamines do not treat stuffiness. Nasal steroid sprays and antihistamine sprays are most effective.

See your ENT: I presume your allergies & sinusitis are affecting your voice. Vocal issues can arise from a variety of problems including allergies, sinus infections, vocal overuse, abuse (singing too loud or out of range), laryngeal reflux disease, vocal cord polyps, and more. You will need to clarify exactly what your symptoms are and your ent/laryngologist will be able to help you and your voice.