I’d actually say a little of both, but mostly whatever you’re most comfortable with. As with everything, the first time is usually a confusing mess and a lot depends, not just on the character’s mental reactions, but also based on who they are fighting and what that person’s skill level is. In some situations, against opponents who don’t know what they’re doing, a character can have those few moments in which to panic, flail, and lock up before the trained instincts finally kick in. They’ll probably be okay if they’re training hasn’t been botched, which it can be. Especially if they’ve never connected their hits with an actual person before.
If they’re fighting against someone who does know what they’re doing, then the time they have suss themselves out is astronomically shortened.
It’s always important to keep in mind that just because a character can hurt someone doesn’t mean they’ll be mentally prepared for it or for the fall out that happens afterward. It’s good you’ve pretty much figured that out. Our brains are hardwired first for survival and second for a preservation of the species, most people, even some trained fighters, have a hard time actually committing to hurting others. The brain controls the body, so if the brain won’t or doesn’t want to connect and injure the attacker, then the body won’t either.
So decide: do you want your character to win the fight? It’s okay if they don’t, it’s okay if they get pounded. They might learn more from that.
If they do win, how do they feel about it afterwards?
Combat is a messy, uncomfortable, and dangerous experience. Your character will be dealing with a lot of emotional fallout even just from the statement “It wasn’t like I thought it would be" or “I hurt someone else", how do they feel about that?
As a general rule: always go with what you know, there’s more emotion to pull from and modify that way. I’ve had plenty of experiences where my brain just clears and I know what I need to do, then do it in situations where everyone else around me has been locked up by panic, but that’s not everybody. It might be hard for you to write that sort of surreal experience if you’ve never had it happen to yourself and that’s okay.
There aren’t any real right answers here (with the exception of select few cases surrounding professions) and writing is pretty much always subjective. If you’re already labeling the instinct as cliche, it’s probably a good idea to go another route.