Partner possibly polyamorous?! What to do?

Hello!

I'm a woman in my late 20s and have been in a relationship for 10 years. I have anxiety and depression and have been on medication for the last five years.

A couple years ago, my partner met someone and became good friends with her. Couple months in, he started having feelings for her. In the meantime, we were thinking about getting married. Before the wedding, I was to travel abroad for a fellowship for two months. When I was away, my partner's friendship with this person grew stronger. Since she lived in another city, he travelled to her city a couple of times and once, they also spent a day together at a hill station. After I was back from the fellowship, we were to marry in less than a month. I was unsure about our future together, so we had a couple of conversations around his relationship with this other person. He said it was 'more than friendship' but without the physical/ sexual aspect. I was uncomfortable with this but since he kept me in the loop about all their meetings and since her partner was okay with her relationship with my partner, I tried to be okay with this special friendship. 

In less than a month into our marriage, I discovered one day that my partner had exchanged a few sexually explicit messages with someone else. This disturbed me to the core. I told this to his sister and the three of us had a long conversation about this. My partner was extremely sorry about the messages, apologized and said this would not happen again. However, this incident triggered something within me. I started feeling that maybe he is not monogamous, that he might be polyamorous. He likes to flirt with different women or likes to joke about doing so. He is proud of the multiple women who came into his life, showed interest in him or tried something with him. He however shares everything with me. I remembered another incident from when I was in college. He had texted one of my friends that thinking of her aroused him. She did not respond to this and he was the one who shared this with me. I was uncomfortable with this but did not say anything at the time. All this led me to think that he may be polyamorous and that might mean things maybe won't work out between us. He however didn't want to label himself anything and was unfamiliar with the term itself.

A few months went by and slowly life assumed a kind of normalcy, but his relationship with this other person remained a source of my discomfort and anxiety. I know I can be insecure and I always fear him leaving me, but having these triggers in the past led me to trust my feelings, despite me not wanting to be jealous, possessive or controlling. So I did not tell him explicitly how his relationship to this person was making me feel, in spite of it continuously affecting me. 

In the following two and a half years, he kept having deep, emotional exchanges with this person. Something akin to romantic kept developing between him and her. They had regular phone calls and he visited her once in a couple of months. By last year, we had had a few conversations about my discomfort with his relationship with this person. This started making him anxious as well. He was diagnosed with high blood pressure and he thought it is due to the stress in the relationship. I kept asking him to seek mental health support but he didn't show much enthusiasm for it. Finally, in November, when he was visiting her in her city, she took him to a psychiatrist there. She sat in with him for the consultation. The psychiatrist gave him some helpful tips and felt reassured after the visit. Him and her, also had a conversation about my depression, my discomfort about their relationship causing him stress. When he returned home and told me the details of this visit, I lost it. I felt like multiple boundaries were crossed. I felt betrayed.

Since then, I haven't been able to talk to him properly. When we did talk in December, it came up that maybe we are not right for each other. That gave both of us significant pain. We spent January and February together, in deep discomfort and pain. Our parents also came to know that we were struggling in the relationship and tried to help. In one of those conversations, my partner and his dad seemed to say that my mental health was making things worse, which made me feel really bad and attacked. 

I decided I needed my own space and moved to another city, where it's easier for me to work. I know I need to take time to heal, since I've seen a lot of pain in the recent past. I need to work on myself so that I am able to state my needs clearly and assertively, since my fear for speaking out has created confusion in the relationship in the past. But while setting up my own home and living independently brings joy, it also hurts. I miss him and I know I want to make things better. I just don't know how. We have had so many conversations where nothing has seemed to resolve. In fact most conversations have resulted in both of us feeling bad. I'm in individual therapy and we have tried couple's therapy, but my partner is not very enthusiastic about it. In fact of the five sessions we have had, all have made him sad, angry or both. I don't know what the way forward is. I don't what to ask him going forward. I would appreciate all the suggestions and advice.