Q1. Here, I think all of them are right to use, but I want to know if there's any difference in nuance.
Playing computer games (by myself / myself / alone) can be fun for a certain amount of time, but it usually becomes boring.
Q2. I'd like to know if 'The fact' can be used as a subject here, and I know I can use 'alone' here, but I want to know if the others are possible to mean the same thing. And which one makes the most sense: after that / after a while / at some point?
The fact that I have a girl friend can be great for a certain amount of time, but (after that / after a while / at some point) I start to think I wish I was (alone / by myself / myself).