I kno I am a young guy and have not been in a whole lot of relationships, especially anything for that amount of time, but when I was dating, I still hung around and flirted with my other girlfriends. Just because you are with someone does not mean you have to stop hanging around all your other girlfriends. That rule also applies for women. Just because they are with you does not mean they should stop hanging around or accociating with there male friends. As long as you are not sleeping around and sh*t it should not matter, otherwise it is called insecurity. But if accociating or hanging around other people causes or leads to doing something stupid, and affects the relationship, the friendship should be cut off before anything happens.
With regard to punctuation and grammar please read some Cormack McCarthy. Not to start a fight but we have a bald brother in pain and sometimes when people are in pain their absolute last concern is spelling and punctuation. So the poor guy comes here to get a few words of advice and gets some shame instead. Let's reign in our high horses, gents.
Rocko, I hope that you can understand that this whole thing is not totally about your fiance. Part of the problem here is about you, too. Not that you are the problem, don't get me wrong. What I'm saying here is that if something like this is bothering you this much then it is not totally about your fiance flirting on the internet. It is as much about something that happened in your life at some time that causes you to have trust issues with this person that you want to have a relationship with, or creates a disbelief on your part that you are sufficient to maintain the relationship.
Check out some counselling before the wedding, seriously, to make sure that you have all of your ducks in a row. Maybe you will find out that this girl is not the right one for you or maybe you will find out that you, yourself, are not ready to get married to her or anybody else at this point in time. But the important thing is to check it out.