I understand that you want to make it work and I can totally understand wanting to fight and hold on to this relationship OP.
Some things to consider for yourself, as right now you are thinking in purely terms of you and him but you really need to be a little selfish and just have a little think about you!
What his relationship like with OW? She was ex, do they have history, were they close, or did he just sleep with her a few times?
Did he leave OW to get back with you? How does OW feel about you? Does she know about you? Is she likely to be bitter about you and you relationship or is she amicable about it all?
How far along is OW? How involved does she was your DP to be in this pregnancy/child? How involved does he want to be? Will OW be ok with you being involved with this baby? If your involved with baby then that means your DD will be involved with baby, how do you think she will feel and how will it affect and impact her life?
Really think long term, not just about when the baby is born but way past that to when child is 7/10/15 and comes with his or her own feelings and wants dad at school play/parents eve/pay for driving lessons/help with uni fee's/first house/wedding etc all these things WILL have and impact and you, your DD and your relationship.
Right now your thinking immediate future like how your going to be upset when he goes off to a scan or when he's out in the dead of night attending the birth of his child. Your going to have to watch him being excited about becoming a first time dad with a baby that's not yours. Your going to have to sit patiently on the sidelines until the mother feels comfortable with you being involved with this baby/child, be prepared for a long wait. Be prepared to sit at home while he attends birthdays, christening etc, be prepared to share him at Xmas, Easter, Father's Day and any other significant dates or occasions. This is the reality of what you will be dealing with and whilst it's good you want to try and make it work but if thinking about it is hard, actually doing it will be much much harder that you can even anticipate.
Good luck!