what exactly is " the fact" and "the fact It ever had been" mean in this context, please?
Which was probably the only honest thing my dad said all night. After that he was quiet. He'd try to plaster up a smile now and then, but man, underneath it he was broody. And I was feeling kind of bad for him. Was he thinking about the good old days playing in a band? I tried picturing him in cowboy boots and a cowboy hat, with a guitar strapped across his shoulder, playing some old Willie Nelson song.
He was right — it just wasn't him.
But the fact that it ever had been made me feel even more like a stranger in a strange land. Then, when the night was over and the Bakers were piling out the front door, something else strange happened.

the boy feel left out in this dress-up dinner between the two families, but why "this fact made me feel more like a stranger"?

Question

what exactly is " the fact" and "the fact It ever had been" mean in this context, please?
Which was probably the only honest thing my dad said all night. After that he was quiet. He'd try to plaster up a smile now and then, but man, underneath it he was broody. And I was feeling kind of bad for him. Was he thinking about the good old days playing in a band? I tried picturing him in cowboy boots and a cowboy hat, with a guitar strapped across his shoulder, playing some old Willie Nelson song. 
He was right — it just wasn't him. 
But the fact that it ever had been made me feel even more like a stranger in a strange land. Then, when the night was over and the Bakers were piling out the front door, something else strange happened.

the boy feel left out in this dress-up dinner between the two families, but why "this fact made me feel more like a stranger"?

Answer

What if your father came home one day and announced he was Superman and started flying through the air? This is not the father you knew and it would seem very strange to you. You might feel as though he had betrayed you by not being honest with you about his secret identity. 
Cowboys are or can be very different from a highly educated suit wearing business executive.

Answer

It just means that the person he is now is not at all like the person he was then. They are so different it does not seem possible he would have ever been a guitar playing cowboy.

Answer

"He was right — it just wasn't him." - The dad is no longer that person who used to play in a band.

"But the fact that it ever had been..." - But he was that person when he was younger, probably before the son was born.

The fact that the son never knew of this significant part of his father's life made him feel more like a stranger in a strange land. He felt that he didn't know his dad.