So sorry to hear that you're in this situation. I was in a very similar headspace a few months ago. My husband left the day before I had our second child and things had been bad between us for a long time before that.
All I can say is I feel less raw now and I feel more optimistic about my children's future. There are still really difficult days and I still worry about the impact on my boys of growing up in a single parent family but I no longer believe that we've wrecked their lives (which I did for a while). In a way I think that having to pretend to be fine for your children like you are doing gets you through because you can't just hide in bed and wallow in self-pity - you have to keep going.
There are already some positives for me. I like that I can focus 100% on my boys while I am with them. I don't have to consider my ex and his preferences and moods and our relationship. I can make decisions in my children's best interests alone. We have really lovely bedtimes now where the toddler and baby come and snuggle up in my bed and we all read together. I've started making plans for all the things we're going to do together in future. I've found I have more support from people than I thought I would. I feel now that I will get to a point where I am mostly happy and when we first separated I couldn't imagine ever being happy again. I think many people go through the same range of emotions.
It obviously depends what your partner is like but my husband was so little help with the children and so disengaged with parenting by the end that I don't feel that the children have lost much. If you have a partner who has been a good and engaged parent hopefully he will continue to be if you separate and your little girl will be fine in the long run.
I've read a lot about the impact of divorce / separation on children and some general parenting stuff since we split up. It makes me feel that I'm doing something positive to help my children through this. I don't think separated parents is ideal for children but in our case I think it became inevitable that we would split up at some point and it is probably better that it was sooner rather than later.
I've also really tried to stay civil with my ex (hard at times as I am so disappointed with his attitude to the children and how he has treated me). I think that's even more important when children are very young as you will be in each other's lives for a very long time and will ideally be able to go to events etc. together.
It sounds like you're feeling some regret that your child might not have a sibling. I ignored warning signs when I got pregnant with our second (though my husband wasn't anything like as bad at that point) and wish in a way that I had left at that point before bringing another child into all of this. I love my baby and don't regret him but also feel so sad and guilty that his dad left before he was even born. Plus single parenting two is going to be much harder than one would have been. So it may be for the best if you separate before having a second child.
My only other thought is whether you have exhausted all your options (e.g. counselling if appropriate) with your partner? The first year of having a child is very tough on most relationships and some people do have a very rocky patch and come out the other end. Personally I will never forgive my ex for all but abandoning his children but I think it's probably easier to get closure and not feel guilty on your children's behalf if you are sure you've tried everything you can to make it work.
Hope you have a better night.