I hate my boyfriends best friend - advice needs!

So basically me and my boyfriend have been together for nearly two and a half years now and we really do have a great relationship. Whenever it's just us two we get on like a house on fire our only problems start when other people are involved. Particularly his best friend.

Now the weird thing is, I don't actually hate her - when we're together we have the best times we're actually VERY alike. The problems start when he's around her he's like a different guy, she makes him laugh in ways I never could and I get this overwhelming feeling of jealousy and I hate myself for it.

But it's not unjust, when it's the three of us I feel like a third wheel, I feel she's very jealous of our relationship and wants him all to herself and isn't happy that I've 'taken him away'. Not only that I also feel she isn't a very good friend, for example he couldn't tell her he was gay - he didn't have the confidence to talk to her about it which is something I just don't understand. When I was finding myself my first point of call was getting advice from my best friends.

As well as this she has previously told him she thinks he can 'do better' and when I asked how he responded to this he proceeded to tell her that I didn't like her either. Fair enough but I was very upset that he didn't defend me or disagree with her in any way. I feel like he prioritises her feelings over mine, for example he once cancelled on me because she had been caught cheating on her boyfriend with a work colleague in her family company (to me that's a problem she should've dealt with on her own!)

I just need some advice on how to handle this, I never want to come between them but I also feel like I should be his first priority when it comes to the two of us. I know I would put him before anything else because I love him that much, and I do make an effort with her. We went away for the weekend (us 3 and one of her mates) I send her birthday messages and cards (none of which is reciprocated). I feel like I'm trying really hard to befriend this girl for the sake of my relationship but she just wants to tear us apart and he doesn't see what's happening. Am I crazy?