Relationships post partum are hard.
Why do you think that post partum relationships are hard?
After delivery, men and women will no longer feel attracted to each other.
There is a lot of stress and uncertainty that comes with being a new parent and it's understandable that a relationship will experience strain after birth.  If you feel as if you need help and support with your relationship, you should reach out to friends and family or even a couples therapist.
I feel really hurt that I don't feel attracted to my partner. I'm looking for someone else to have sex with.
Nobody is perfect. if you are really struggling I think it might be helpful to reach out and talk through your feelings before acting on them.  Have you told your partner how you feel?