OK. I'll explain. Let's use an analogy.
You are getting yourself dressed.
You put on underwear, trousers, a shirt, socks, shoes, and then to finish it all off, you pick up a cummerbund. And you put it on your head.
You sort of know what a cummerbund is. You're just not quite sure how to use it.
But instead of leaving it well alone, you use it wrongly.
That's what people do with apostrophes. They could just ignore them. It may still be wrong, but we'd know what you mean. But, instead, they insert them in a random place and hope for the best.
My advice to you, if the rules for apostrophes are too difficult, is just don't use them. They are, after all, over-used rather than under-used. In a given day, if you don't use them at all, you will probably be right more than you are wrong.
If, on the other hand, you'd like to use them correctly, the main thing you need to remember is.
STOP USING THEM TO MAKE A PLURAL!!!!!!!!!!!