Everyone here had given you such good advice already.
Maybe try with you on top first, that way you can control the speed and the degree of penetration.
Lots of lube,
But most importantly is take it slow, maybe dont do penetration the first few times, just play?
Sex is so much more than the physical thing, your emotional and mental state impact on it, and having a baby and all that goes with it is easily enough to impact on all parts of it.
*hugs* it will get easier just takes time.