Hi,
I write the following with the best intentions, so although it will sound harsh, I wish you the very best.
Ok, I don't blame her for cheating on you.
Dude, you have been engaged for 2 years.
TWO full years.
Over six-hundred days!
My lord... why??
I bet she thinks it will never happen.
I re-read your post and I'm slacked jawed.
She has no respect for you... because you have no respect for you.
Why should she??
As to your girlfriend, it is clear that she could be a very "sexually active" person... and that is fine!
But, truth be told, you are not working out for her in that department.
It is possible that deep down you do not like her
that much.
So... you're going to leave her?
And why can't you stay in the same house?
I think I missed something.
Your friends that have flats are in the forces... so... I don't get it.
Why don't you keep on staying in that house?
Is she the owner? Or the one that pays rent?
If she is neither, just break up with her, and get her stuff out of your new room.
Again, I woudn't blame her.
Yet clearly, you and her are not on the same track.
My 0.02