When you say "I've met someone else" is it really to that point with this 3rd party? I'm confused based of your post where you stand with this other person.
Regardless, I've been on both sides of this coin. The inevitable infatuation can just be an excuse to get out of a relationship you don't want to be in anyway. If so, then just end the LTR.
If you are dedicated to your LTR, then keep it at light flirting and make the other person aware you are taken. If it progresses to a line that you know you shouldn't cross, then you have to make a choice. Drop one or the other.
Regardless, I've been on both sides of this coin. The inevitable infatuation can just be an excuse to get out of a relationship you don't want to be in anyway. If so, then just end the LTR.
If you are dedicated to your LTR, then keep it at light flirting and make the other person aware you are taken. If it progresses to a line that you know you shouldn't cross, then you have to make a choice. Drop one or the other.