I hate to be the bearer of bad news, but sadly, chances are you won't get the daughter. taking custody from the biological mother and giving the child to a non-biological parent is an extremely difficult case.
MysticSpade: I truly feel for ya bud. I recently went through a very similar situation myself. My ex wife left me for an ex con she met at the bar she worked at. It sucks. I too raised children that weren't mine. Funny how attached you get to them. But the sad thing is, you have to let the child go. If you don't she'll use the child to get things from you. She WILL use that child to get you to help her out.
I know what you're thinkin... But she won't do that... WRONG! she will. she'll manipulate you through the child. "Sky needs this. Sky needs that. and I can't afford X for her." She won't be different. Sadly, you'll fall for it. We all do at first.
Leave Ash as a part of your past. If she does come back to you, you don't want her. She cheated on you. May not have been physical, but it was cheating just the same. If you take her back because mister X-Box Live guy didn't turn out to be the guy she was dreaming of, within a few years, she will find someone else to cheat on you with. Probably even be so bold as to cheat in your own bed, while you're off at work, and accuse you of cheating on her. That's how a cheater is.
But you won't believe me. She'll want to come back and you will take her back. because you will think "Tommycat doesn't know me or Ash, She's different. She's the perfect person and will be the exception. It's the meds. It was a 'One Time Thing™' and it will never happen again." Yep I said that myself... well not about Tommycat, but it was another forum several years ago. I didn't want to believe that someone I invested 2 years of my life with was just like what that moron on the forum said. But he was right. eight years later I caught her cheating again, and this time it was in my own house, and in my own bed.
But it's not all bad news. After she's gone, you can focus on improving yourself. Do something YOU want to do. Live how YOU want to live. and eventually you will find someone else. Trust me. And she'll make Ash look like a worthless layabout piece of garbage.
"I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it." Thomas Jefferson