best friend and boyfriend- advice needed!

If you could please read this and offer up your suggestions I would be really really grateful. I need an unbiased opinion. Sorry it's so long! Sooo... I was out with my best friend a while ago at a bar when this boy came over to us and started up a conversation. He offered to buy us both drinks and while he went to get them I commented to her that I thought he was quite fit and had a good personality. My friend on the other hand told me that she wished he would leave us alone as she thought he was annoying and ugly. I left the bar before her as I had an early start the next day and a little later he gave her a lift home as she had become separated from the person who was supposed to be dropping her back. The next day I spoke to her on the phone and she told me nothing had happened between them but that she had left her cardigan in his car and asked if maybe I would get it back for her as she wasn't interested in seeing him again.

A few weeks later I was at another friend’s party when I spotted this guy. We got chatting and ended up kissing. The next day I called my best friend up to tell her what had happened. I was totally surprised when she told me that she was a bit annoyed! She said that actually she did fancy this guy and had been thinking about him a lot over the last few weeks. She also said that the only reason that she said she though he was ugly was that she was drunk and that makes her be a bit harsh. Thing is, when she said it she’d only had one drink. Also, when I spoke to her the next day after we had met him she was totally sober and still seemed completely uninterested and didn't even want to see him herself to have the cardigan returned to her. Also, we had chatted several times since then and she'd never mentioned him once. I pointed out all these things but we ended up having a mini argument about this which resulted in us making up but her saying that if I ever saw this guy again not to tell her about it because she would just get upset.

Basically, I really didn’t know what to do because I did want to see him again. Eventually after talking to my Mum and a couple of friends I decided that (even if she didn’t realise it) the only reason that she thought she liked him was because she knew I did. This girl is prettier than me and much more popular with guys but I have noticed every time someone shows more interest in me than her she tries to win them round to liking her even if she doesn't fancy them. So, I decided to meet up with this guy and not tell her (as she had asked me not to if I ever saw him again). I didn’t expect anything to come of it but turned out that we got on really well and so have been seeing each other for the last three months and meeting up pretty much every other day or so.

Even though she told me not to I feel like I really should tell my best friend about it now, as it feels horrible to keep such a big thing from her. Do you think this is a good idea? I have arranged to meet up with her when she gets back from a festival for a drink to tell her but I have no idea what to say or how to make her see it from my point of view and not get upset. And finally, was I right to start seeing him in the first place or am I being unreasonable? Thank you in advance!! xxx

This is one of my biggest hates about women. Ugh. Hate it.

edit: the bold bit.