We overestimate our short-term ability, but underestimate our long-term ability
Learning another (human) language is like that.  Almost anyone can do it.  It requires no special skills, training, or experience.  There are some tricks that you can use, but mostly it's just an enormous amount of work, not particularly challenging work either, that you need to persist at for years, even decades.I'd like to think this is common knowledge, but I have many times talked with people who hope to learn French to fluency in six months, and people who are convinced they could never learn it no matter what.  Both types are terrible with estimating their abilities.