If you spread your feet wider, far beyond shoulder width if you have to, and slide your stopper out to the point where it is pulling your string gently taut, it becomes easier. This lowers your bow, and brings your more powerful muscles into play. Otherwise, you end up having to pull with shoulders/traps/arm. This makes it a bitch to use the other arm to set the string.
Crouching down, with feet wide, and knees bent, lets you use the power of your hips, legs, and back to bend the stave, rather than having to pull up with your arm. If you SLOWLY use the power of your torso, it becomes a much more controllable thing.
Then you can concentrate on just gripping the handle, and gently sliding the string outward with your other hand, until it hits the groove. I find the sweet spot on each bow is different, and I have to crouch/spread more, the higher the poundage.
These days, I mainly use the old Step-through, but doing this with a one pocket stringer, I can string up to my 82# 70" Grevy, with no real issues, just a bit of care.
Just carefully place that stopper, keeping a little pressure so it doesn't move, then slowly straighten your core and legs so it remains always under control.
Oh, wear shoes or boots with a solid sole, too. That also helps a little!