Look..real talk here.
This situation is going to come up again and again as long as you are best friends with a past sexual partner. Most people just do not want to think about the person they are dating fucking someone else. But when a past sexual partner is a significant part of your life, it makes it really hard to turn off that part of your brain that gets irrationally jealous. Your SO's will always wonder if you are prioritizing this man because he is your best friend, or because you used to sleep with him. They are always going to wonder why your connection to your friend always seems to take precedent over your connection with your SO. Basically, get used to relationships ending over this because it will happen again at some point.
And let me make something very clear to you. Your boyfriend is not being irrational. He is not being petty or narrow minded. He is acting like practically any other man would. He JUST NOW found out his girlfriends best friend, whom is very important to her and is a significant part of her life, is someone she used to sleep with. And to top it all off, he JUST FOUND OUT that you hid that information from him for the duration of your relationship. Why? Because you knew he probably wouldn't be interested in dating you if he knew. So you lied. Your entire relationship was based on the foundation of one single lie.
And instead of you being open and honest from the beginning and giving your boyfriend time and space to come to terms with this revalation...you expect him to be ok with it immediately after you have been lying to his face every single day for almost a year. You are asking him to trust you WHEN YOU HAVE BEEN LYING TO HIS FACE EVERY SINGLE DAY FOR ALMOST A YEAR.
Then, you had an opportunity to prove to your boyfriend that you could prioritize him over your BF...and you chose to prioritize your best friend. You could have called your BF, explained the situation, and he could have ubered to the train station no big deal. But you didn't. You allowed your already irrational jealous and unstable in the relationship boyfriend to witness you prioritizing your best friends feelings over his, your best friends needs over his.
It is very clear that you share a connection with your Best friend that no other man is ever going to be able to compete with. Its also very clear that honesty is not something that is very important to you otherwise you wouldn't have lied to someone you claim to have cared about.
You say you don't believe that your SO should become the most important person in your life simply because you are dating them. You are right. They do not have to be the most important person. But you do have to make them a priority. And you do have to prioritize them over ex's.
In the future...when you start dating someone, tell them right away that you used to sleep with your best friend. Include your new SO in your relationship with your best friend so that they can see for them selves...that the two of you are nothing but friends and that he has nothing to worry about. Because the only way for someone to accept this scenario is to be honest with them from the very beginning and trust that your actions speak louder than your words.