If you can get the pads onto the stirrups, put on a fence of something similar and hammer down the edges so they are flat to the bottom of your stirrup. If you don't they make a noise that upsets many horses.
When my pads would not go on the stirrups, I used a hoof pick to pry them open a bit more and then used the hammer to lock them into place.
I bought a pair of cheese grater pads a few years ago and could not get them into my stirrups no matter what I tried. We eventually figured out the stirrup pads were 4 3/4", which were larger than my stirrups. It may be something you want to consider. I know how irritating it is, believe me.