My wife is 5'1" and 120lbs - she has had no troubles with my XDMs or Glocks. The only thing she doesn't like is the 1911, because the plunger in the front is a bit tough.
It sounds like you just need to learn how to use leverage in your favor.
Hold the gun in your right hand in a shooting style grip. Point the gun to the left and get a firm grasp on the top of the slide. Force your hands together to move the slide to the rear, rather than sling shot style - my wife has trouble with sling shot style, too. With this method, you're using your upper arms and shoulders to compress the spring.
Kind of difficult to explain, I suppose, but look up YouTube videos. I'm sure there are some demonstrations.
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