OP,, two or three head or chest colds a year and a bout with the flu once a year would not concern me.. That's about how I roll but my wife works with kids and I am In and out of Hospitals five or six times a week with my job.

I consider this normal, unchanged for more than a decade now.

You said, "sick more often and more intensely",,, and, "This has been an ongoing thing for the last 15 years or more".
Starting from my level a pattern like you speak of, "ongoing for 15 years", would put me near death....

You better find a GOOD Doctor....