I've been in a twelve step recovery program for quite a few years and one thing has become pretty obvious to me
eople who have issues of their trust being violated do not heal quickly, even when they do decide to forgive.
In Brian's case he hasn't got just one loss of trust issue to deal with, but two. He's lost trust in you AND in his former best friend. Those are heavy, heavy issues to deal with
You say that you tell him you love him. Brian remembers you telling him that many times before. But to him, those expressions of love have been sullied and they aren't going to mean much for a long time (longer than a year).
You need counselling to uncover and deal with why you almost jumped in bed with this other guy. That would do more to heal the relationship than constantly telling Brian you love him.
SLE