[Early 20's couple] she kissed another guy! Is the love still there?

I've being going out with the same girl for almost 5 years now. Obviously we are way past the puppy love stage but I'm now wondering if we genuinely should be together anymore... It's something that's being playing on my mind for a while now.

As specified in the attention grabbing she recently drunkenly kissed another guy. But to her credit she owned up straight away and I know nothing further happened. If ever there was a reason to break up with someone then that would be it. But she swore over and over she still loves and only wants me. To be fair I believed her. She's not a good liar...

The thing is, I have been wondering if things were right even before the incident above. I mean, we argue like any other couple. But for the most part we get along fine. This is my first serious relationship so I'm not sure if this is just the way relationships are or if they should be more... fulfilling, for lack of a better word?

And I don't claim to be perfect by any means, I'm not. And if anything I may be going through a patch where I am slightly apathetic towards life. I'm struggling a bit for daily motivation. This doesn't help matters. I'm not going to say I'm depressed because that would be disrespectful towards those who are actually depressed.

I guess I'd like to hear the opinion of people who are more experienced in relationships or have been in a similar scenario to offer how they went about their issues? I wouldn't say the relationship is toxic by any means but it just seems to be gradually losing its spark.

Just to provide some extra information (if it helps) both of us were never exactly highly romantic or emotional people. So we probably aren't as lovey dovey as most anyway, which was never really an issue...

Anyway thanks for hearing me out.

TL;DR; she kissed another guy but even before that I felt the relationship has been in decline.