My 8 month old baby has been sick with a cold for a month!?

He got a cold on July 1st. I took him to the doctor on July 6th and they told me to do the whole humidifier, tylenol, and suctioning the nose thing. I did and then I suctioned his nose too much and I irritated it. He got a really bad cough and I took him to the doctor again and he gave me amoxicillin to get rid of the green stuff... yuck... and now he's got a cold again! What am I doing wrong?!? Is it the air conditioner? Please help me!