Para vs Por

Para vs Por
 

I thought I was getting my head around para/por, but appreciate any help with why they are chosen in this context, why would it be incorrect to use them the other way around?

Sabes que por mi la felicidad es muy importante, pero, para mi culpa no vamos a ser felices

The first underscored word should be 'para'; and 'para mí,' the phrase. This is a prepositional phrase denoting purpose.

The other underscored word could be 'para' or 'por,' depending on what the speaker is trying to communicate.
If the speaker wants to say "but it's my fault we're not going to be happy," 'para mi culpa' is used. This gives a reason.
Using 'por mi culpa' yields "but because of me we're not going to be happy." This phrasing conveys 'as a result'.