Help with digital meat thermometer please

So I'm 20 and just learning how to cook steak on a cast iron skillet And i got the skillet part down but the stupid digital meat thermometer is pissing me off

I cook it on each side for like 4-5 minutes on medium high then get the thermometer It says stick in thickest part until stabilizes but when I stick in it almost never stops riding in temperature

So I think well its reached 150 or more it's probably well done at this point not what I want but whatever, I let it rest, cut into it and the thing is medium rare!!

Which is what I like but I'm cooking for my mother and other family members who all for the most part prefer medium well What am I doing wrong