Thank you for the feedback.
It appears to me that you might be feeling betrayed and threatened. You might also be feeling like the relationship is threatened as well. When you say you feel angry and that he owes you some loyalty, I interpret that as you feel hurt and betrayed.
Anger is a stoppling off point to hurting. We often say we are angry and stay in that place, because it hurts to much to experience what we are really feeling.
From what you described, I get that he is not that much into you. How could he love you when he is entertaining the other best friend. It is one thing to have a best friend outside the relationship, but to be spending as much or more time with the best friend as you, indicates that he is not fully vested in you or your relationship. The dilemma of course is that you care for him and have been supporting him socially, and perhaps in his professional life as well.
You can not control this person, and you would not want to be in a forced relationship. That would not bring you the satisfaction and devotion you deserve.
Your action is a hard one to do. My recommendation is to pull back, give him space. Talk to him about you and him and the relationship. do it over candle light dinner. Make it sweet, no argumentativeness. Just a sweet talk where you tell him how you feel, and ask him how he feels about you. Ask him where he things the two of you are going. If he is vague and uncertain, leave it at that for the moment. Finish the evening on a positive note. THEN on the next day, at breakfast perhaps....ask him for a moment.
Tell him, look, I do not feel I am getting the attention and devotion I deserve. I feel you are not into me as much as you are into the best friend, and that you feel he is not ready for a more committed relationship. Then get his reaction to that.
Then in that evening, bring up the subject again, and this time let him know, that you will be moving out. You understand how he has the need for the other friend, but that if you are going to get the love and devotion you want and deserve you realize you need to find it somewhere else.
The whole frame work here, is that you can control you. YOu can extract yourself and treat yourself well. you can cut your losses with dignity, while respecting him and his needs. This will put you in the position of power, even though it may not seem so at first. The reason is, that it will now be up to to him to decide what he really wants out of a relationship. You are now free to seek the relationship you deserve and need. If he does not come back around, that is good, because you may then have found a new relationship; or he may come around, and if you are still available, you can then decide to give it another go. If he comes back around, do not go back without getting the expectations settled first.