I Am a Stepmother Forum & Chat Board | First Day of School

My stepson is 4. His mom and dad are recently divorced and I still feel she hold animonisty towards me. She had her father until he passed away, my man had a split family so he somewhat understands. His ex wife had a son from a previous relationship - this guy didn't care about the kid though so he somewhat played dad. I have a split family and have dated a dad who had a chill ex-wife, you respected her motherhood you could be present for anything. Fast forward and now their little boy is about to start his very first day of school. They are going to go to the teacher meeting together, and drop him off at his first day together. I don't mind this it is their son. I don't want my man to miss out on anything especially since he is home (he is home for 3 weeks and works offshore for 3 weeks). Like I said my ex had a baby mama who would let me be present for just about anything, and now I'm dealing with pretty much having to be hands-off. I have still got to help him pick out his backpack with his dad, and lunch pale and shoes for our house. I just I guess I feel bad knowing I can't be apart of the big day and feel somewhat jealous. I have had several miscarriages and don't even know if I want me and him to try for kids. I don't know how to feel, she got upset because I was apart of splitting the school supply list. I know that she is jealous, and probably still not over him or the fact another woman wants to be apart of her sons life - though I have been around him for quite sometime. I was wondering if anyone has experienced this and advice of getting through it without causing issues with me and him because honestly I don't want to say anything because I don't want him to worry about me when he's home during such a exciting time for his son. Beyond how to deal with this, is there any other advice on helping ease her mind knowing I want to be everything but her replacement. I have always respected her, and never said a cross word about her to the kid. Any advice would be appreciated.