Nice topic
I would tell myself not to expect your relationship to remain exactly the same. I will have to share my partner's love with my child which is awesome, but is also sad at times.
I would tell myself that not sleeping would bring out the worst in myself in so many ways. That work would be a battle, my love life would be a battle and being a good mom would be a battle.
I would tell myself that no matter how much I wanted to keep my own identity and take care of myself, I wouldn't be able to as much as before. I wear the same shoes all the time like my mom did even though I promised myself I wouldn't.
I would tell myself that my birth plan was just that. I would tell myself to be prepared for plan B and C.
I would tell myself that one doesn't always get to choose how many children we want. Some have twins when they want one and some have one when they want twins.
I would tell myself that I was a real pain in the arse childless person. Telling parents what to do. Shocking.
I would tell myself that no matter how hard I try, I will end up sounding the same as my mom.
I would tell myself that my partner is an equal parent and should be able to be just that. Especially in the beginning.
I would tell myself that things always work out as they should.
I would tell myself to get help with a traumatic birth on day one and not wait till 6 weeks. By then the damage is already done!
I would tell myself to join a forum during the pregnancy. The support is priceless and the friends you make will be friends for life.
I would tell myself that age three is as bad as everyone makes out it is.