Thanks a lot to both of you. I'm new to this website and it seems like I can't change the status of my question from unsolved to solved since you guys left a comment, not an answer...? Anyway I got it now!

I think you mean, "I didn't realize what I had in store.¨ "What I had it door" makes no sense in English. The more correct expression is "I didn't realize what I had in store for me." This is an idiomatic expression indicating surprise.

She's saying "I didn't realize what I had in store". https://idioms.thefreedictionary.com/have+in+store In this context, it means that at the time she didn't realize how much work would be needed to plan a wedding. She could have said, "I didn't realize how much work I would have to do". Having something in store usually means having something planned for you, or having something waiting for you. Another example: Before I went to university, I didn't realize how much work I had in store. I hope this helps.