I had that problem before.
It's pretty simple....someone taught me a technique that helps to trick 'em! ;)
Simply stand like you're going to get up, then jiggle the saddle a bit, to see if the horse move. If the horse moves, halt it, & then try wiggling the saddle again...(you'll have to tighten up your girth of course after doing this fully), & see if the horse moves.
Well, it might not work for you (keeping the legs square might work better for you!) but it worked for me. It kinda teased the horse, & when I finally got on, didn't move at all!!
They think when the saddle moves a bit, it's time to GO!!
But if 'ya trick them, & wiggle the saddle when you're not getting on, they most likely won't do it again.
But I don't know your horse, so you might be better off trying the other techniques. ;)