Sometimes merely and just can be interchanged but quite often they can't. For example, "just" can also mean righteous, like a "just king". It can be a way to express time, like "I just arrived". It implies you arrived shortly before. If you were to say "I merely arrived", that would imply you only arrived and did nothing else - it does not give the reader any sense of time. And so on.
So basically, "merely" means "only that and nothing more or less". "Just" can have a variety of meaning depending on the sentence it's in.
@Virtuoso You didn't interrupt at all. Anyway i am almost done with script. now the worst part i have to memorize scirpt and vocabularies. and above all i have to translate topik questions which are hell difficult 😭