I went through a similar experience... my then-fiancee went from daydreaming about us as grandparents some day to "shutting down" emotionally once I got pregnant earlier than expected. The stress I endured over his on-agin, off-again partnering style was severe, so that it was in many ways a relief when he abruptly moved out (& in with an ex who had recently moved here...) when our daughter was 2 months old.
What helped was giving up on "the dream" of a decent relationship with her bio-dad, & instead focusing on my health & my current & future relationship with her. I went out & bought some books that tell you what your baby is doing every day of your pregnancy, mom-to-be journals, etc.
I managed to survive working very part-time. Since I couldn't give her a traditional nuclear family, I wanted to at least give my daughter a stable home life. It paid off, big time.
My daughter is now a happy, healthy, sweet and empathetic kindergartner. We live very frugally (in a tiny 1950 bungalow that my ex was going to renovate & flip but that has now been home sweet home for the past 6 years!). We shop at consignment & sell outgrown clothes at consignment too, so we rarely have to pay for clothes at all.(I can give you more "making ends meet" tips if you want... I have tons!) The important thing is, it can be done, and YOU CAN DO IT! You sound like someone with a lot of natural empathy who will not only be a great mother, but who will get even more out of it than you give. (I was given up for adoption as a newborn, & while I had a difficult childhood and I often wish my biological mom had either kept me or stayed involved in my life, I am grateful she didn't abort me.)
Good luck, enjoy pregnancy & motherhood despite the unsupportive bio-dad, & keep us posted! :-)
P.S.- I thought my ex might be sociopathic after all the lies I uncovered, but now I feel he's got Narcissistic Personality Disorder. Google it & see if any rings true...