constantly sick...

is it normal to be constantly sick while you're at a young age? it seems as if all of my family members start of that way on my mom's side of the family. i used to always be sick as a child. where if i didnt have a fever or the flu i was still sick with a cold or severe allergies. im now turning 22 this July and I'm still constantly sick. Usually everyone in my family gets a better immune system by age 15. ive been struggling with work cause ive been so sick lately. i still attempt to go in but for the past 2 months they have been sending me home early because its getting so bad. my doctor has ordered a blood test from me and im about to go in for another appointment because i think i have a sinus infection and i would rather have antibiotics. im currently taking robitusin during the day, tylenol cold and flu at night, vicks vapor rub throughout the day and night, and ive been using a sline nasal spray before and after bed. nothing is helping. idk what to think at this point.