Michelle,
Your feelings are very normal and you are wise to seek support and encouragement from your fellow Smoms. The reality is that the role of stepmom is hard and often we can be overtaken by emotions that we did not anticipate.
One of the biggest struggles for stepmoms is our own expectations. Given that we inherit many people and relationships when we say "I do" we are going to be impacted by those people and relationships in ways we may have never anticipated.
Your dreams and expectations of married life with your husband are being impacted by his daughter and new granddaughter. You are wise to recognize these emotions and seek ways to work with them.
First, I would suggest explaining your feelings to your husband during a time that you set aside (don't do it in the heat of the moment so to speak). Explain that your feelings are neither right nor wrong but they are how you are feeling. That you understand he loves his daughter and granddaughter and that you seek to not take that away from him but rather let me know how you are feeling and suggest ways that he might help you.
You may have a "no kid/grandkid" discussion time at the end of the day. A time where you two connect but there is no kid talk. Think of a fun/silly code word to use if one of you gets off track. That way no one is "correcting" the other and it can be a fun playful way to keep the conversation flowing just about the two of you.
As far as Facebook and photos are concerned, explain to him that you don't want him to stop taking photos of his granddaughter but rather would like if he would continue to take photos/posts of you also.
Explain what is on your heart to him.
And think of something you enjoy and when you start to have feelings of resentment, first recognize that it is normal and then think about something that brings you joy and/or do something that will take your mind off things (ie go for a walk, listen to fun music, read a book, etc. And continue to find ways to connect and nurture your relationship with your husband. Don't let this create distance for you.
Know that your husband loves you and is also happy for his daughter and new granddaughter.
Thanks for reaching out. Inviting our other awesome smoms to add some encouragement for you.
xoxo Heather