You need two things in working order before you mount. Your horse needs to have a stand still cue and know how to move off of pressure.
First i'll teach the horse to flex. Stand at your horses side and with a halter on, just take the slack out of your leadrope. Make sure that you are just holding the pressure and not pulling. If you're worried about it hold the upper part of your arm against your side so your arm won't move back. You want to have the pressure outward vs straight back to make it more simple for your horse. As soon as your horse gives her nose and keeps her feet still remove the pressure and let her straighten her head. Wait a moment and repeat. She'll move around in circles at first most likely, just hold to pressure and wait for her to stop. She may also just lean on the pressure, again just hold it and don't increase it. As she gets better, raise your arm so it'll be closer to the hight of where your hands will be when you ride. Repeat on both sides.
She should also disengage her hindquarters. Shorten your lead rope so you've taken the slack out as you make an arc to her butt. Either use the end of your lead or a whip to then tap ( or twirl) the air, slowly increase the pressure to a tap, then if needed a whack until she untracks her hind feet. You want the inside hind to cross OVER the outside. If it doesn't keep the pressure until it does happen.
You horse also needs to know how to lunge. Grab your lunging setup ( mine is a rope halter with a 12 foot lead. Great for teaching a horse because you can still be close. When I'm lunging a horse that knows how to I have a 23 foot line), stand in front of your horse and point the direction you want her to go we'll use to the left here. Your left hand holds the line, it should be short enough that you've taken the slack out and are just applying a bit of pressure. Cluck ( or whatever your verbal trot cue is), now use a training stick or lunge whip and twirl it on the other side of her neck ( her left, your right). If she doesn't move after a few twirls, tap her on the neck.
Some horses move before the tap, that's great. As soon as she moves forward on a circle, leave her be, quit pointing and quit twirling. Other horses start backing up, don't increase the pressure, just follow her back with the same amount of pressure on the halter and tapping her. You don't want to increase the pressure because she is already trying something, increasing it would just make her run backwards faster. Again as soon as she goes, leave her be. Other horses still are pretty dull and you'll have to tap on her neck progressively harder until they do try something. By breaking it down into steps you'll end up with a horse that will lunge off of only you pointing.
Repeat until she gets it and you don't have to tap her to get her to move forward. Once she gets to go forward on a circle Then you can worry about keeping her going, again slowly increase pressure till you get what you want. If she pulls on you, just bump her nose into the center of the circle then release. You don't want to hold pressure because it will give her something to lean on.
When you are ready to stop pull the leadrope across your hip then put pressure on her hind end to disengage it. You may need to shorten your leadrope a couple times for her to get it.
Now that you have a solution to your problem you can start. You won't be using a mounting block for this, lower your left stirrup as much as you need to. It's MUCH easier to teach without a block. Once she gets to stand then adding in the block is simple. You either want split reins for this or to stay in your lunging setup. You want to be able to move quickly to moving your horse around. If your reins are singular reins that go over your horses head, don't use them. If you are in split reins, half hitch that right sucker up so it's out of the way.
Now before you mount, be critical of how you do so.
This is a great video showing how to mount properly, and can be done enlgish or western. Many people mount by instead standing perpendicular to their horse and pulling themselves on. This will cause the saddle to want to roll and your weight is too far away from the horse. Even the best trained horses will start walking off after enough of this to keep their balance.
So, flex your horse and stand at your horses side. Is that good? Yes?
Raise your left foot. Is that good? Yes?
Go ahead and put left hand in mane and right hand on horn and raise your leg. Is that good? Yes?
Put weight in the stirrup and step up, leaning your weight over your horses back. Is that good?
When you find a place your horse no longer wants to stand still, immediately put her feet to work, being quite firm. You don't need to do more than a few circles as it looses meaning to the horse.
Then stop your horse, flex and begin your checklist again. Within a few mounting attempts she should stand stock still if your timing is right and you prepared your groundwork correctly.