To fall out of love = you or someone once loved a person/thing, but you don't love that person/thing anymore.
To fall in love = you or someone started to love a person/thing.
By then = at a deadline of a specified time.
To be used to = something is very common for you, so you don't even think about it. Example: I'm used to wear these shoes.
You wear those shoes frequently, so you're used to them. You don't even think about them.
It is a question of keeping your room clean = the minimum that you can do is keeping your room clean.