She more moves her butt away and then moves to the side. Sometimes she does go forward.
First thing is to make sure her bit is ok. Want you to do the following on a slightly loose rein. Make sure you are not pulling on her mouth at all.
Have you tried moving the mounting block next to a wall so she has to go between the mounting block and the wall?
This is a temporary solution and a first step to training.
Find a place where it is difficult for her to move away. (If she is similar to most drafts, she will not make a huge effort to get away, just taking a step or two to the side is enough!)
Do not reprimand her or try to hold her still. Tell her whoa and if she moves just pick up the mounting block once she stops and put it next to her again. Important to remain 100% calm and act like you have all day.
Then get on. Sit for a minute, maybe even give her a little treat from the saddle.
Get back off. Repeat. Repeat, repeat. End of lesson one. Do not end the mounting lesson until you can get on without her moving. Reward her heavily for standing still.
Later same day, or next time, repeat mounting lesson. Move block to different location. Repeat mounting lesson.
Do an easy ride. Return to mounting block, repeat lesson several times. Put horse up.
Continue to mount several times each time before and after you ride, with treats given. She will quickly look forward to mounting as a really fun thing to do.
This is not a method for a professional. This is a method a beginner can easily do on their own or with a tiny bit of help in the beginning that can make mounting a pleasure instead of a struggle.