mMy relationship

When we just got together, he happened to move to the city where I live and started to lived by himself to begin with.

I had lived with my family, but I gradually started to limove in with him gradually because we hadspent a lot of time together.

Since we were college students until last month, we had a lot of time to spend with,together, and did things like watching movies, hanging out together, eating out, and drinking often.

But, since April, he has been super busy because he has becomes a member of society.

"... he has become a member of society" sounds either like a you're talking about him becoming a very high-level important person like a lord, or that you're joking. If you're joking, you can add an exclamation mark at the end of the sentence to show that. Otherwise perhaps you could say "because he has gotten a job". Everyone is a member of society :)

He is a manager of his personal fitness club and an instructor as well.

Does he own the club? If so, your sentence is fine, except you'd probably say "He is the owner and manager..." If he works there but doesn't own it, you could say "He is the manager at a personal fitness club" or just "He manages a personal fitness club and is an instructor as well".

He works as an instructor from morning to evening and after that, he startdoes office work after 12until after midnight if it finishes late.

Moreover, he has recently been renovating his gym by himself recently to make another space by separating two spaces/a room, which enables him to instruct two clients at the same time.

You need a subject for what he is separating.

I was going to Australia on a working holiday after I graduated from college, but the plan was spoiled because of the pandemic of cCoronavirus pandemic.

And tThis situation will continue a little bit longer, so I gave up on going to Australia once and started job hunting in Japan.

I know I have to get used to this situation early, but it's kind of difficult for me to do that.