Good morning everyone 
Baby lovely, you really are 'going through it' I read back at about 6.30am and I've been lying in bed and contemplating relationships, children, parenthood, our parents etc etc Firstly, most teenagers are vile creatures (especially girls!) I for one was really awful, I spoke to my Mum like a piece of shit (similarly to my DD1 does to me now) when you're 'in it' it is so hard to see it from the outside so that you can reflect and put it into prospective.
A daughter's first love is their father (as a son's is their mother) They can do no wrong (this extends to their new partner sometimes too) there is nothing you can do about 'the step mother is nicer, better, prettier than you' syndrome. Their relationship will go up and down over time, mainly depending on how your X is with your daughter, if he's behaving in the way that she wants him too it will all be rosie until that changes, which it will in time. These bits you won't know about because you will only hear the good bits. Especially when DD is comparing you to her.
There is quite a good book about teenagers (sorry if you already know about it) 'How to talk so teens will listen & listen so teens will talk' (it is American, so a few Americanism) but I found it really useful.
In relationships I do think, whether your married, had partner for years, had kids with someone or not, that the woman is the one that holds it all together, they are the backbone if you like. From entertaining the DC, to making sure their uniform is sorted, their emotional needs, booking holidays etc etc And as women we have this expectation of our man that it will all be equal, perfect, balanced and when it isn't there is a huge disappointment. I for one spent years thinking it will be alright in the end because he knows my ultimate reasons that we've decided to be together and we're working towards this goal together. As women we are so determined to make things work. My brother pointed out recently (when we were stuck on a car journey together) that had my X-partner not left me, I would still be with him today and do you know what, he was absolutely right! My X did me a favour because he took the decision away from me and left. That was nearly 5 years ago and I was devastated, to say the least, and so angry, how dare he, I used to say. Anyway, I am OK, I did survive, I am much less angry, It is lonely but I know where I'm at, my girls are fine, we are skint (as I have not had a penny from X)
Like you said - this is not a dress rehearsal, this is it
I'm going to post this now before it gets lost - sorry that it is so long......