Oh Nix, I'm so sorry you are feeling so low, please call your g.p first thing tomorrow and ask for an emergency appointment. Tell them everything, especially that you want to keep your baby! They can put you on anti depressants, they can also refer you to the community midwife and possibly counselling!
One of the best things you can do is take your little girl out and power walk around the park, or the beach. Gentle exercise can increase your endorphins and make you feel better. You have decided to keep your little one, so buy a pregnancy magazine or a pregnancy app and look up your due date and your babies development at however many weeks you are. Allow yourself to get excited, you are growing a beautiful baby, a sibling for your beautiful daughter!
Take your little one to soft play, or swimming, or the local toddler group. Try to keep busy so that come evening, coupled with first trimester exhaustion you can both drift straight off to sleep. Obviously if morning sickness is awful then by all means give your little one your phone/ iPad/pop cbeebies on your telly and curl up in bed with your princess by your side. If your little girl is unsettled bring her into bed with you for a short while! You can work on getting her settled in her own bed later on, you are what she needs right now!
Do you have any other relatives who would be supportive? Any siblings, cousins, grandparents or friends who you could visit and get support from? If your ex gets abusive about your choice to keep your baby, tell him he had his chance to stop this happening back when he was actively trying for a baby with yo! He chose to make this baby, despite shagging someone else!
Please dont compare yourself or feel like OW has won, if anyone is a winner here its you! You have seen his true colours! He is a nasty, cheating, pathetic scumbag! His bit on the side will have to live with knowing that even when things are good and supposedly going great (e.g settled down and trying for a baby) he could be living a double life shagging someone else!!!