So here is the story. I have been with my best friend (let’s call him B) for 2 years, where he has done everything for me, I trust and care for him deeply and vice versa. However about a year into my friendship with B, I started dating a girl. I hit it off really well with her from the start, I asked her out a couple months later, we hit a rough patch and took a long break, then we got back together last year. I know this was probably not the best thing to do, but I never officially told B that I was in a relationship because I didn’t know how he would respond and I feared any change to the best friend dynamic. However, this stance changed when I got into a fight with my gf, as I then went to B for help on what to do and revealing my relationship in the process. He said he knew all along that I was in a relationship, but never chose to say anything out of respect of my privacy, yet was not prepared to deal with this new information. Of course, he was a champ and helped me restore things with her. After that, things were going along well until over the next several months, we kept getting into a series of fights about how he feels my attention and effort since then was going more into my gf rather than B. Each time I resolved it by doing just that, shifting my efforts to B, but he says it is temporary because I’ve been not attentive to him for a few weeks when he needed me (which I admit he is right I made a big mistake that I did twice and deeply apologized and he eventually accepted only the first time) even though I remain committed to my best friend B and bettering myself, and I have not repeated this bad behavior since. But because I screwed up twice, B progressively has built a lack of trust and comfort in me as he assumes any time that I do not spend with him is spend with her (not true at all, I have a family too and have shown proof I was doing something with them but denies it). I’ve asked B to me how we can fix things and for some help on how to do it, which gets a response from B saying I have to think of that myself. He says I’ve been acting weird and turning into a stranger to him ever since I restored things with my gf. I’ve tried talking through to him with a plan about how I can make him feel more valued again and be there for B when he needs me. I was not prepared for B’s response. He said, “If the plan includes your gf in your life \[at all\], you can’t have me at all. You must choose between me or her, but not both.” Followed by (not a verbatim quote but it is what B has told me before) “It is hard for me to trust you or go back to you when you go to her, and I am not willing to negotiate that. I do not want to feel disregarded or hurt again. I want to have control of my feelings from you, I do not want to feel emotionally unavailable, and you must make your decisions clear by solving the main problem (B feels an emotional imbalance from me between my gf and him)”. I was taken aback, and I asked if this means me being single is the solution, to which he said “No, you don’t have to be single, you just don’t get to have me”, and no further details were given to me about this last statement beyond that.
More context is that B has had a similar situation happen with B’s previous best friend where he has dumped a best friend since B’s previous best friend got a gf, but the difference is B’s previous best friend didn’t even bother trying or caring. That’s not me. I care and I love B deeply, but I also love my gf and I would never want to do any of them wrong. I have been relentless in extensively contacting and showing B my appreciation for him this whole time, and I have even been emotionally vulnerable. B has also never been in a romantic relationship before and he keeps thinking my gf will replace him, which is false. I have told him that I value platonic relationships too and the length of our best friendship with its prior satisfaction proves it.
However, he’s handed me an ultimatum by tomorrow, where if I say I choose him, I must explain in detail exactly how I will solve the main problem and what I will do. If he thinks it is not good or elaborate enough, he will dump me forever. If he does think it is good, he is willing to forgive me for everything and go back to normal. Of course I choose him. What do I say in my response to truly convince him?
TLDR: Best friend helped me restore my relationship with my girlfriend after a fight, but since then, he feels neglected as he says I’ve focused more on her despite me shifting my effort to him. Best friend now gives me an ultimatum: choose between him or my girlfriend. I choose him but need to convince him I’ll prioritize our friendship. How do I respond convincingly?