I want to make it clear that this is not related to my relationship, it's just something that I've observed.
OK, so most of us have (or have had at some point in the past) a best or very-close friend of the opposite sex. You hang out together, are eachother's platonic "date" at weddings and dinners when everyone else seems to be in couples, and you snuggle up on the sofa to watch movies. You try out new restaurants/bars together, provide eachother with advice about the opposite sex, and are the comfortable shoulder to cry on and moral support when the other gets dumped. You can and do talk about anything and everything. During lonely periods, you've maybe even toyed with the idea of the two of you as a couple - heaven forbid - but realised you're missing the X-factor to be anything more than great mates.
And then he meets someone special. You're excited for him, prepare him for dates with her, listen as he tells you all about her. You can't wait to meet her - if she's good enough for your best mate, she's got to be wonderful. As time goes on he, understandably, has less time for you and more time for her. And that's exactly how it should be.
But here's where some women get it wrong. It's at this point that you should BACK OFF. He has found someone special to hang out with, to be his date at weddings and dinners, to snuggle up with on the sofa, to try out new restaurants with. He doesn't need relationship advice (unless he asks for it), and you certainly can't go phoning his mobile at midnight when your date from match.com turns out to be a loser.
You can still be his friend but you must respect his new relationship and his new girlfriend. He's a boy and you're a girl; it cannot be the same between you; them's the rules!
If you try to continue things how they were - wanting to hang out with him all the time, being affectionate with him, calling and texting on a daily basis - you're going to seriously annoy his girlfriend and this puts him in a difficult situation; either he risks upsetting you by having to ask you to back off a bit, or his girlfriend has a strop every time you throw your arms around him in public.
So do the decent thing; respect their relationship, and back off. Get on with your life. Everyone knows that relationships can be hard at times and he doesn't need you throwing a spanner in the works and spoiling things for him. And remember that what goes around comes around; when YOU meet someone special, do you want his pretty, flirty, best friend sitting on his knee at parties?
Think about it.