Setting in pages in between existing pages messes everything up

So I got a problem, or actually more than one.

Thing is this: I made a 2 page master page layout which I used to create 14 pages of (7x2pages).

No I found out that I have to add a third page to all this 7 double pages (7x3).

First I thought I could add a New (third) page to the existing masterpage, but when I do this and create another page after the exisiting double pages,

the first page of the masterpage is what I get, not the Third page. So I would have to create Three New pages to get this Third one, but I cant do this because I already overruled the master page on all pages with different texts and pictures which takes a lot of time to do once again.

So I created a New masterpage instead. Problem With this is when I add this master page (or any other page) in between the different pages, all pages after this getting messed up, With text and Pictures flying around, being placed doubled and such.

I Guess I understand what causes this problem, as the content on all pages getting moved to the masterpage they are not designed to be in (page 1 jumpes to the 2 of the masterpage when text frames and Pictures are different). Question is simply how I can fix this without having to start from the beginning?