When your friend is being unkind, ask her why she is being unkind to you. Perhaps you are doing something that is irritating her. Ask her for as kind and honest an answer as she can muster. Listen, and then decide if this is something that can be worked out or worked on. If not, and she is just insensitive, then get a new friend. Life is too short.
A best friend doesn't take this much work. You either get along, have fun together,are not jealous of other's friends and have pretty much the same moral standards. They aren't ever mean intentionally towards each other, and as you get older you love their kids, and pretty much raise them as cousins. Also, you can trust her with your deepest secrets, without fear. You can disagree and get back together like nothing happened. Be willing to apologize if you are wrong. Let her vent about her ex- husband, but you can tell her when she is exaggerating her point. Calling her out, and being called out by her, helps you both get closer, and grow.