Living together with someone is difficult

I live in a house that I share with several flatmates. 
Recently, one of the guys has been making me angry.
In my opinion, he is a really messy person.
He always makes a mess after showering or cooking food, etc.
So, today, I told him "Please keep the house clean. We share it!!"
However, he didn't agree.
Rather/Instead, he made excuses for everything and complained to me. 
I was really shocked.
I realized that I had been upset for a long time and told him today.
But this selfish guy made me upset !! (I am not quite sure what you mean here.  I think you are trying to say that you had been thinking about this for a long time and finally said something.)
I really regret accepting/allowing him into this house. 
It's really difficult to live with someone who has different habits.