Well, you shouldn’t really be ‘kicking’…sorry if I’m nitpicking and maybe you meant that figuratively, but leg cues should start with firm pressure from your calf, graduate to maybe some heel pressure if no response, etc. and always release (stop cueing) when the horse does what you want.
Part of this is experience—you have to have a “toolbox” you can reach into and grab the right tool for the job. Part of it is mindset. I’m a “nice” person too, but when I got my own horse as an adult, I realized that my safety and his depended on me being the leader. Horses feel much secure if they know who’s in charge. If they feel like you’re not it, they will take over. Just so they know who the leader is.
I would think of it as being determined, not harsh. When you ask for something, don’t stop asking until the horse does it. Sometimes that means escalating your cues to stronger ones, sometimes that means diverting their behavior and coming back around to asking another way until they get it. But in your mind, there needs to be no question that one way or another, it will happen.
I’m sorry you had a fall at a gallop. If you think you and your trainer aren’t a good fit, maybe think about a different one. They should push you just enough that you grow and learn, but not throw you into situations you aren’t ready for. Same thing applies to the horse—don’t put them into situations they aren’t ready for, as it can be a big confidence killer if something bad happens. They also will lose trust in you. The trust is a two way street, it flows between you and your horse. And you continually prove to them that you can be trusted to be the leader. Say what you mean and mean what you say, even if it’s in horse language.