To get the animation to stop you should look into anim notifications, or as frostdog mentioned; montages.
I don’t have any montages in my animations, so I’ll post one way to do it with just animations.
Setup the key you want holding down in your character blueprint, with a variable that can be set.
Next. add a notify in your animation.
Expose the play rate on your animation in your AnimGraph, and plug in a variable.
Finally in your AnimGraph Event Graph add the notify event like so. Basically what this will do is pause your animation while the key is held, and then continue playing it once it is released.
Hope either this or FrostDog’s set you on the right track 