My best friend has found the next best friend... Not sure how to react, but I am now ignored, after over a handful of years.

If she/he found a new best friend and is now ignoring you, she/he isn't worth it anymore, trust me its happened to me many times before, she just told you by ignoring you is that she doenst want to be your best friend anymore, take this by a person who has had this happened many time before now, if you talk to her it might as well go back as it was, and keep happening! Hope I helped

Often "best friends" change, but friendships can last.
Often distants, job changes, moves makes it hard. You may go from talking several times a day to several times a year.
Then newer friends from work or new groups have priority.

Well it either her/he doesn't like you very much and is more popular and take advantage of your best friend ,the best thing to do is to keep in touch with he/her or if her new boyfriend is nice then getting along with her/he is the best thing to do.