He’s in love with two? 30F&38F

How do you guys go about the person you love with everything in you and have for two years being in love with you and another? I am working on getting comfortable with the situation. Me and the other girl used to be on good terms but not anymore. And everytime they're together my anxiety skyrockets. How do I proceed? I want him happy but this is not an easy feat at all I'm really struggling with getting used to it. Especially with the other always hurting him. We have been in multiple Triads and I wouldnt mind if this turned into one but as it is now, I'm so uncomfortable. Will the comfort come with time? For more info, I'm Pansexual , he is Pansexual polyamourous, she is bisexual but isn't into Cis Women (which I am). She has said that could change but I doubt it. M/37, F/38, F/30 (me). I have known/ been on and off with him for 2 years and them about 1 year.