Scientific explanation correlating with NoFap :-
Dopamine = Happiness Hormone.
Brain secretes dopamine when a person does PMO.
With NoFap going on, which means no PMO, which means no Dopamine secretion. Which ultimately means No Happiness.
Thus giving a sense of loneliness, low self esteem etc.
To deal with it :- you must build self-motivation habits, self improvement habits, enjoy your own company habits, make yourself happy bexause no one else is going to do it for you habits. You are the reason why your brain is asking for PMO induced dopamine, so you have to substitute it with something better. Teach your brain to be happy and satisfied with what you have and what you are going through, loneliness is also good, one can use it to improve self qualities (thus contributing towards long term + future satisfaction and happiness)
All these are long term process, which leads to the secretion of seratonin , which is long term reward system.
Also Dopamine = Short term reward system
So when a person is done with the PMO act, no Dopamine secretions thus he feels guilty and depressed.