just to add to the above. In times past, a more mature, respected friend or family member might have had a quiet chat with your partner: probably to just to explain the impact of the arrival of the new child on the both of you and how your partner now needs to be supportive and strong for all 3 of you.
That of course was in the days before social media as it is now, but I think it is still a valid approach.
Another 'old fashioned' view would be to say that now you are a mother first and that your body will get back into shape in due course, ( I know that there is more to it than this).
I think perhaps you could chat about your concerns to your health visitor as well.
Of course there are groups like the NCT Natioal Childbirh Trust, or Mumsnet where you can meet and chat with other new and not so new mums and get support and strenght.
Wishing you well,