Does watching porn and not masturbate lead my brain into flatline? (I am not very sure about this)
Yes; you may go into a flatline.
One day is guaranted but may extend upto 7 days.
To avoid a flatline from your life i suggest you keep engage yourself in challenging tasks & activities
progressing in physical excercises.
progressing in Running.
Progressing in studying.
Learning new skills.
Listening to music.
Focusing on your current & long term goals.
Joining outdoor games club.
Sitting with your family
Tire yourself whole day to get a good night sleep.
Going in nature for a walk.
Smiling to strangers.
Eating your favourite food.
Watching a movie with a friend.
Making new friends.
Making a good circle of friends.
Going out for a vacation.
Working hard in every task .
Working for others.
Having emotion of love.
Meditating with eyes wide open while doing your work.