I'm so sorry you're going through this.
First, you need to reach out to as many friends and family as possible. Call your midwife and explain - she will point you in the right direction.
Next, speak to your doctor. I'm not sure if you can be given antidepressants whilst pregnant but they might have something they can offer you. It's a slippery slope with hormones and to go through this heartbreak whilst pregnant is not easy at all.
Lastly, have faith that this will pass. You'll need plenty of time to process and grieve and that's OK. Even if it takes a year or 2, that's OK. You're going to adapt to having a newborn (not sure if this is your first), which will be challenging for the first few months, which is why it's so important to have a support network in place.
That man never deserved you. Remember that. You can give all the love and devotion in the world, but if it's not reciprocated, that person does not and will not deserve to be graced with your presence nor consideration. He's made his bed, let him enjoy it because he's the one who will be missing out on a beautiful family, not you. You and your child will have a wonderful life together. It'll be tough at times but it will also be full of unconditional love and laughter, lots of laughter. ❤
Stay strong, call the doctor, call friends and family.