Hi, everyone - I'm new here and love how much wisdom and support is on offer; I hope I can borrow some right now 
This might seem a bit trivial and not that important in comparison with some of the truly terrible situations a lot of people face but it's upset me and I'd like some opinions.
Setting the scene a bit: I only have 3 close friends - (I'm generally quite introverted and don't open up to people that much) but they are really good, can phone at 3am to cry sort of friends. I work with one of them and we have a colleague who we both weren't that keen on but got on with ok as it's a small team etc. Now, this friend of mine has become really friendly with this colleague lately, going out every weekend, talking non-stop at work etc. We've stopped spending any time together because she's always either out with her "new" friend or is doing family stuff, which is obviously fine. (I dislike clubbing and don't drink so tended to ask her for brunch, coffee, etc)
I suggested one day that the 3 of us should go out for dinner after work: they both nodded non-commitally and a few hours later my friend texts me saying tat she and this colleague are going out for dinner and I can join them "if I want"! This sort of thing has happened more than once and I'm upset at being left out for someone who she agreed only a few months ago was unpleasant to say the least!
I want to tell her that I feel upset and betrayed. I want to tell her that she ditches me as soon as someone more exciting comes along (yes, this isn't the 1st time), I want to tell her that she is being dishonest by befriending someone we both disliked and now is apparently great.
I've read this back and I sound like a drama queen. But I am so upset, I feel like I've lost a friend and said friend doesn't care.