Robbland, welcome, your post is seen and you are heard

ps apologies if this isnt in the right section - i felt it was most relevant
You're in exactly the right place, and you deserve kudos for reaching out for help in this excruciating time in your life. We're here for you.
While I'm usually over on the Family Law board, I often check out other boards for learning, so, here I am welcoming you!
I dont really know what to do, what to say, where to go, and just feel like i need some support.
These are the hardest relationships with the most challenging people. As you've experienced, people with BPD-type behaviors can have black-and-white, win-lose, me-or-them thinking. It's not healthy, mature, or appropriate for your GF (ex-GF?) to make ultimatums about "me or your daughter".
Robbland, are you 110% sure that this possible pregnancy is "yours"? Sadly, pwBPD (people with BPD) might not see fidelity in the same way that non-BPD folks do. If it "feels true" that she needed to be with someone else, well, that may be how she saw it. Or, if she "feels pregnant", even if she isn't... well, that's something else to consider. These are wild, far-out ways of thinking and coping that might not seem normal (because they're not!) to us. But, as she might be trying to use "a pregnancy" to rope you back in... just be forewarned to be discerning as you consider what she's telling you.
Kudos to you once again for caring so much about your daughter. That is the right priority -- she's the child, your GF is the adult.
You're doing great things even in this struggle. Loving your child, asking for help, trying to do the right thing.
Hang in there and keep posting. I know others will be along soon to care for you, too.
kells76