What should I do? Why do I feel this way? How do I know where to put boundaries between them?

Hello and thank you for posting your question in this forum. This is a rather difficult and complex concern but we will attempt to help you deconstruct it as much as possible.

It sounds like your boyfriend got a new friend who is a girl, in his life. You may be correctly feeling somewhat threatened by their relationship. Relationships are primarily built on trust. Trust must be maintained in order for a relationship to work. No other element is as important as trust within the context of a relationship. That trust can be eroded for any number of reasons but a primary one is experiencing and sort of strife, in the context of your relationship. If you are already experiencing difficulties and a new person enters your life or your boyfriend’s life (like you have described) it can often be tempting to get involved with that new person. This is not to say it’s a guarantee, but statistically it can often happen in the ways you have described, for many couples. The new person can become a source of emotional support and this can create a different outlet for frustration or negative emotions. This might encourage an emotional connection that may be seen as a replacement for the current relationship. 

In order to keep your relationship strong you must seek some couple’s therapy. Doing this will also help your boyfriend to continue to focus his energies on your relationship and to invest his time into improving your relationship. Perhaps he and you would open up to a couple’s therapist and discuss your issues fully as well as why things have not been going well (as you have noted).  

You will want to explore your past and events which occurred in your past, as well as your potential future together and where you generally see your lives going. You will also want to explore your family structure as well as his family structure. And how family relationships may have influenced your current decisions and approaches to life. You do not necessarily need to do that on this platform - you can also seek that sort of support in your local community. It may actually be more beneficial for you to do such exploration with a therapist in a local office as that can often be more meaningful when doing couple’s therapy. No matter what path you choose, it will remain very important that you do this for both your peace of mind and to address any concerns with the relationship. If you do not address such concerns, there may be trust gaps which grow in the relationship and these will eventually push the both of you away. Additionally, your boyfriend may be encouraged to use his current situation to his advantage and to build trust with his new friend. Again, this would end up usurping power from your current romantic relationship and it is not an outcome which will be beneficial to you, in the long run.

Hopefully your boyfriend is willing to go on the couple’s counseling journey with you and to seek both an improvement in your relationship and better mutual understanding. I hope you will pursue this for the benefit of your relationship and to help yourself through these relationship difficulties. Thank you again for posting your question in this forum.

I wish you good luck in the future as well as good physical and mental health.