Hello I Have Had A Cough For More Then 3

Hello I have had a cough for more then 3 weeks I either phlegm which is a yellow colour or sick sometimes I can't breath as coughing to much when I sleep I will wake up gasping for air I have been to the doctors got some amoxicillin but this has not helped I am tierd as it wakes me up every night what shall I do is this serious please help