Kissing itself isn’t a sin, it’s what occurs as a result of the kissing which can be problematic. If kissing causes an onset of lust which you can’t snap out of, well, that’s a problem from a Catholic morality standpoint. If kissing leads you to either masturbate or have sex before marriage, this is also a problem from a Catholic morality standpoint.
And really, that’s true for anything with your significant other, not just kissing. If hugging your girlfriend is generating a lustful response from you that stays for awhile, then you probably need to stop hugging. If hugging is leading you to other immoral actions, then put the brakes on it. There’s a Scriptural precedent for this; if your hand causes you to sin, cut it off (metaphorically, I’m certain).
I mean, it’s great that you’re attracted to someone, and I wouldn’t say a brief moment of lust is a mortal sin. But dwelling on it, seeking it out, and feeding it? That’s not a good idea if you wish to subscribe to a life of Catholic morality.
Also, consider; there’s some lines you cannot cross back from once you move past them. or at least it’s very difficult to do so. Consider this when deliberating on which actions are appropriate.