What to do when you found out that the guy you thought was your fiance is getting merried to another
I am so sorry to hear about this situation. I am sure that this was extremely heart breaking and life altering.
Unfortunately, it sounds like there is not a whole lot that you can do. If your partner has made up their mind to be with someone else, there is not much that can be done EXCEPT FOR focusing on taking care of yourself during this distressing time.
I would highly encourage you to reach out to trusted friends and family during this time so that you are not alone. Being pregnant can be stressful, and when you add in stressful life events on top of that, it can feel extremely overwhelming. Who are the people in your life that you trust and can lean on?
I think it is imperative to focus on self-care and relaxation. As difficult as that may sound, it can really help your mental and emotional health along with your baby's health. Some people really enjoy engaging their senses to work on self soothing. A few examples are taking a relaxing shower or bath. You can engage multiple senses doing this; the scent of your soap, shampoo, conditioner, bath salts, etc. The temperature of the water and how slippery the soap is. The sounds of the water. Looking at the bubbles/suds or the shampoo/conditioner in your hand. Another example of engaging your senses is creating a really comfortable, safe place in your home that you can go to, to relax. For some people this is their bedroom. They may make sure that their bed is comfortable, light a candle, dim the lights, put on music... create a very relaxing environment that you can retreat to when you are feeling stressed or overwhelmed.
Another form of self-care will be deciding if this person will be someone that remains in your life or not. It sounds like, because he is the father of your child, your preference is for him to remain in your life but he is not taking responsibility. You may want to check in with your doctor to see what your options are in a situation such as this one. I would be willing to bet that they have been a part of situations such as this one and may know what direction to steer you in.