TL/DR: Monogamous but need advice / rules on how to deal with a crush when it is taking away to much from the relationship, partner can't deal with no contact.

I will start with the fact that i (M 30) have a monogamous relationship and my partner (F 30) actually has no intentions to open the relationship but i believe that this community here can provide better advice than the actual relationship_advice subreddit.

My partner met a man 6 weeks ago on a vacation that she instantlly clicked with. He lives in another country but they quickly planned a visit and started chatting on a daily basis. I was very jealous at first but i tried my best to trust my partner. However i noticed that she was pulling further away from me, from 5-6 days a week we spend together down to more like 2-3.

Two weeks ago she told me that she cut contact with the guy since she thinks she has fallen in love with him and wanted to sort things out. Last week she wanted to break up, after hours of talking we finally found out together that part of the reason she wanted to break up was, that she could not bear having no contact to that guy any longer.

I have had an ENM relationship in the past and know the struggles of falling in love, however current my partner is handling the situation really bad. She was venting about everything she dislikes about me to the guy and it is very obvious that (with her rose-tinted glasses) she sees the other guy as some sort of saint (she coulnd't even tell me anything that bothers her about him).

We finally concluded that she will have contact with him again and also visit him like she planned to do, but she will try to limit the amount of chatting, won't vent about me and there will be no flirting (and there was none so far).

Since you are all experts in handling emotions for more than one person here I'd like to know if you have some rules or other advice on how to balance multiple partners when severly crushing on one.