Like rvby says, I think the important thing is that you have to be realistic that it may not happen, and start doing this as soon as possible. Then, you have to make your peace with the fact that it isn't happening. When people say, oh, it will probably be fine, this is wrong, because for a lot of people, men and women, it won't turn out the way you want it.
It happened to me. It's not an easy process - I have always wanted a husband and children, but it didn't happen. I struggled with it for a long time, but my lightbulb moment came when my female GP (also single and childless) said to me "there is more than one road to take to be happy". And she was right. I started to actively plan for a life on my own - from idealistic topics like travel and challenges, to practical things like my pension. So much time and energy is wasted by delaying things just in case Mr Right (or Mr He'll Do) comes along. When the realisation comes that you aren't having what you want, then there doesn't seem to be a gaping hole in your life so much. It softens the blow a little bit.
The other thing I thought was that part of me hating being single and childless was about feeling left out and alienated when all my friends were settling down and having children when I wasn't. Every BBQ, every party, was all about the kids, and there was me, some kind of maiden auntie. You just have to try and put it out of your mind. Don't go to every b'day, every christening, etc if you don't feel up to it. I did my best mat's kids and that was fine. Don't put yourself through anything that might upset you.
It isn't a quick or easy process. You'll be in your mid-forties, thinking you're over it, going on nice holidays and learning to play the piano, and then you're told you're in perimenopause, and go on HRT, and you think, "oh, this really isn't going to happen, is it?" And it really hits you in the solar plexus. And so you take time to grieve for the family that never was, and you take a deep breath, and you carry on. Because you still have a life of your own to live. There will be a hole there where you wanted a family to be, but it can be a good, happy life, and it's up to you to fill that hole, because only you will find something else that fills it. It might not be a perfect fit, but it will leave a crack rather than a gaping chasm.
I try to be a good auntie, and a good godmother. I don't sweat the small stuff. I sponsor a child in Laos. I take my friends out for the afternoon and listen to their mum woes. Despite not having children, and struggling with it, I'm now in a good place and pretty happy.