Why do you have to be in contact with your metamour to maintain your relationship? If the friendship between you and your metamour is unhealthy that's a problem with that relationship, not a problem with the relationship between you and your partner.
If you are going to break off contact with your metamour have the conversation with your partner first so that he isn't blindsided by it, especially if he gets the fallout from your metamour. Then talk to her and let her know that you don't wish to be in daily contact anymore. Then be strong and don't reply to her texts unless it's something that you want to talk about that is going to benefit both of you.
Everything will be ok in the end. If it's not ok it's not the end.