Why am I always sick?

Since I was only 11 years old, I become sick up to 5 times a year. It increases as I get older, and it's starting to become very annoying. They especially come during winter, but they can come at any time. It's usually a sore throat, stuffy nose, congested head and sinuses, headaches, muscle aches, cold sweats, fevers, and all around feeling like crap. I'm seeing my doctor in about 3 months.