An open letter for help. Do I stay and fight or do I let go?

My boyfriend and I have been together for 3 years and have had a friendship of 15+ years. We’ve had wonderful adventure together and I’d say he’s one of my best friends.

About a year ago, I changed industries and accepted a demanding job. It’s required me to relocate to a new city for 1-2 years. Before accepting the job, we agreed that it was worth it and we’d commute and while it would be hard we’d figure it out. It’s been 5 months since I officially moved and they have been awful. I’ve never felt further apart from him than right now. He admitted he hasn’t been as invested in our relationship recently but after we talked over Christmas was committed.

I’ve been feeling insecure and questioning his behavior because we are hardly seeing each other, we don’t talk about our future anymore, and our paths seem to be going in different directions. He mentioned looking at a condo where he lives and I fixated on that. I keep thinking about it over and over and that it was him moving on to this new life without me. When we talked about it, he told that’s not the case but recently his actions aren’t aligning with his words.

I confronted him about an employee relationship and asked if he was having an affair. He told me no and I mostly believed him. A few days later I caught him lying to me about her. He had spent the day at her house working and he told me he was at his club and had dinner with different co-worker. When I confronted him, he eventually came clean and said he didn’t tell me he had meetings and dinner with her because he didn’t want to hurt me. He seems sincere and kept saying over and over “this is my biggest and only mistake I’ve made. I should have been honest and told you I was meeting with her but I didn’t and I regret that but I’m not having an affair with her. I care too much about the work she’s doing to mess it up.” He said he knows he has to work hard to build back my trust.

Beyond the depressing lack of trust I have in him right now, I also am questioning myself and if this relationship is worth it. I’m 39 and am sacred to have to start over. I’m scared to and honestly don’t want to lose my best friend. I’m scared of the silence of the day, no calls, no texts, no one being there for me. I’m scared to try and fight for this relationship on the chance that he’ll lie again when it’s easier than telling the truth.

How will I ever trust him again? Will I always have this uncertainty in our relationship? How do we/I move on? Either together or separately.

I also wonder in the out of my stomach if I’m being manipulated and wonder if he’s never going to change. I’m scared to find out in a few years that he’s always going to prioritize himself over us. He’s always going to choose to attend the next event or party instead prioritizing time together. Everything I bring up that bothers me, is refuted. Is this manipulation or is this him expressing what bothers him back? I really don’t know.

Two examples:
Issue: Not communicating things that are happening in his life/work makes me feel isolated and that our relationship doesn’t have a future. The response is, “I’m scared to tell you because when I tell you, you use it against me.”
Issue: Prioritization of his time and how he has been picking work over us. The response is “You’re making me choose between you and building my business.”

I’m lost. I love him and I thought he was my forever person but I also love myself and don’t want to disrespect myself for someone who’s not really what I need.