Need help with writing to a girl (Please read story)

Today I saw this girl, a girl that I knew back in my first year of college back when I was 16 eeek!. So a good 4 years ago! When we first met we were kinda seeing each other and it was obvious we liked it each other alot!
It all went downhill one day I cant remember why haha just remember being weak and chasing her(how I wish I knew what I know now back then!)think it may have been something to do with us being young and her not sure of what she liked in guys. Just us both immature and confused? It led to her flirting with my friends to make me jealous as well I remember. But eventually our friends fell out with her, causing me to be in the middle but yet some of the stuff that she said really made me get angry with her more.

So she never spoke to any of us and I eventually stopped talking to these friends aswell, so we all went our separate ways.

I have seen her twice before today, this was at a big gig in London by pure luck should I say, she was in the queue metres back with her boyfriend(now ex) didnt think much of it besides her being that girl I knew and fell out with. As I was talking to my brother at the time I did notice she kept pointing and looking my way, I just ignored it and acted like I didnt notice.
Again I saw her at a festival last summer again for a brief moment.

So today I saw her again and she didnt see me. But for some reason I would like to message her, see hows shes doing and what shes done with her life etc. Yes I saw her today and thought wow she looks great! Looks wise she has turned into what id love for a girl to look like haha!

I usually wouldnt ask for help on such a thing. But being the past of us, conflict with friendships, and the immature times of the teens! Im kinda stumped for ideas!
I would like some help in what I should say in my message to her to strike up a convo, something to gain her interest in talking to me. Aswell to show how over the years ive matured like I imagine she has, just something that she wont be tempted into ignoring. I want to make a good impression of the me of today really, so she doesnt sit and think. Your still that boy I remember! I hope as well she isnt like that same girl I remember.

If anyone would like some details that may help out write a better message feel free to ask, ino shes into her fashion, same music and that she goes to the same college as my little brother, aswell we both love tattoos. I noticed as well she has changed her hair quite a bit looks really good, being a hairdresser now its a thing I tend to end up looking at a bit with people haha.
Not sure if I should apologise for the past and things I may of said or done back then.
But if anyone is willing to help, please! Hopefully time can heal past wounds and conflicts. Id rather try and fail then to not try this out at all.
Sadly I would like to just be able to bump into and talk or text her but I only have facebook as a way to contact her.

Thank you :)