Blower motor control switch speed on dash for heat and air?

And I turn on the air or the heater. The control switch does not work on 1, 2, 3 clicks) lower settings). I have to turn it all the way up to as far as it can go, which would be 5. It’s not numbered it just has a color mark like starting small and going to the largest -I have to turn it up to max (5) or it won’t blow sometimes I want it on 4 if it’s getting too hot or too cold in the car and if I turn it just right, which takes time I can get it to stay there to be honest it doesn’t make a lot of a difference but a small difference if I try to turn it to medium speed(3) I will get no blowing out of the vents. Why doesn’t it work in the lower blowing modes and only the higher or the highest blowing mode lately, I’ve been having to put a matchbook or toothpick or something underneath it to hold it up a bit and it does not seem loose at all, and I’ve just been hearing a flapping sound on the passenger side under the windshield Behind the dash like where the wheel when I start the car and it stops after a min or so.. It recently went to a Mechanic that the tow company recommended, the throttle body engine light came on and I think it went into limp mode it had been jerking for a few months and poor gas mileage. I thought it was the spark plugs. I’m a single mom who has cancer and don’t know anybody to fix it so I was just wondering if you guys could give me your best guess on how much it will cost and if it will be fixable on what it is.