I'm so sorry you're going through this. I think your baby will be the light of your life and will give you the strength to get through all of this. Something similar happened to my best friend, her partner left her when their baby was 2 months old, went with a few different women for a few months, then came back. Then they got pregnant again, got married, and he did it all over again when the baby was born. He came back when the baby was 6 months old, had another child soon after, and now that baby is about 1 1/2 and they have finally split up again for good. She has had a long road of learning to stand up for herself against an emotionally abusive, narcissistic, selfish man, and depend on herself, but she is a fantastic and devoted mother and her kids are her anchors. Now she is dating her best friend who is a wonderful caring man who has watched her go through this all these years. Things do get better!
As for your birth, I would highly suggest hiring a certified Doula to support you during labor. They are trained professionals in supporting all kinds of women in birth, and would be invaluable to you especially with all of this emotional upheaval occurring now. They will be with you at your side through the whole thing, will help you stay focused, informed, and keep your mind and emotions in a good place so you can have a great experience welcoming your baby despite whatever else is happening in your life. Depending on the person you may even be able to have them come to your childbirth classes with you, which would be good to get to know them a little ahead of time. Even if you have a relative at your birth a doula is still very helpful as they are trained in helping women cope with contractions and anything that may happen, and also will be added support so your family member doesn't feel as much pressure and can be able to take breaks and get food, nap, etc if needed.
Best of luck with everything and I hope you have a wonderful birth. You'll get through this!!