Finally, everyone in my house is gone just m's gone from my house, and I'm the only one left.

My house isn’t small butexcept for when everybody is here.

NBut now I can do whatever I want.

"but" contrasts with the previous sentence better.

I don’t need to go to the library, or the gym…

I can do all that in this house and no one will get in my way.

That is the best charismas present I could ever get!

"best present I could ever ask for" is another expression one can use here.

I can have my home backthe house to myself!

Finally, everyone in my house is gone jusbut me left.

Or you could say,

My family (or whoever is living with you) finally left, and I'm home alone.

My house isn’t small, but when everybody is hereome, all I can do is stay in my room.

It wasn't a complete sentence.

All I can do is stay inside my room.

I've added this to the other sentence to make it a complete sentence.

I don’t need to go to the library, gym…the gym, etc.

I can do all thatose things in this house, and no one will be in my way.

That is the best chaChristmas present I get've received!