If it does not look right or sound right, it is not right.
I am exactly the same way. I was a military brat, and moved around a lot. I cannot diagram a sentence.
But people ask me to proofread their work.
I can write without a spell or grammar check aid, and I can simply tell when a sentence is or isn’t correct. Of course, I do have a copy of the “Elements of Style” and the Internet for those times I just cannot tell...:)