(I love your name)
One thing you have to remember is that much of how we learn to parent is by watching our own parents. It's not an "excuse", but it's a possible "reason" for our parenting problems. Not that there's one specific "reason" but it's something to look at when we try to figure out why we are the way we are.
(I had a "depressed" Mom who tried to commit suicide a couple of times. Then my dad told me if she succeeded I'D be at fault)
Surely your mom's "condition" didn't suddenly appear out of nowhere when you decided to leave. My guess is that she was depressed to various degrees all along the way and your leaving was possibly the "straw that broke the camel's back" or her "reasoning" for letting go of her control. Either way, you DIDN'T force her to turn to drink.
But regardless of all the stuff we have to put up with sometimes you just have to "take the bull by the horns" and tell yourself "enough is enough...yes, I have issues to deal with BUT life goes on and I'm going to WORK to let the buck STOP here. I'm going to WORK emotionally to pick up the pieces, put them together, take responcibility and EVENTUALLY let God heal me as much as possible."
"Love" isn't always a mushy, visable, steriotypical thing defined only by what we "feel" or see on the outside. It's there for your daughter but it's burried by a bunch of muck and baggage and scars from having carried a HUGE load heaped upon you by your past.
At some point we need to set that "load" aside...somehow. That's where the work comes in. You need to be free to love your daughter on the OUTSIDE the way you know you love your daughter on the INSIDE.
Hey, if you didn't love her it wouldn't even bother you that you had this guilt...because you wouldn't have it to begin with!(the guilt) The fact that this is painful proves that your love for her is alive. Therefore there is HOPE! Sure, it's difficult to convince your heart of the things you know in your head are true. But life in this imperfect world is no picnic!
But with God's help, we can take responcibility and get the job done! And we have so many helps these days to do it...Be it doctors, meds, friends, websites, pastors, prayer, coping skills...whatever it takes. It's ALL out there...