Adults do find English harder than children do. That part is true. What is not true is the reason most people give, which is that we have somehow got worse. The real explanation comes down to two very concrete things, and both are within your control: time, and method.
Why children have the advantage
Children have an enormous amount of time. Even if their method is somewhat wrong, they have the hours to relearn it properly, because eating, sleeping and learning is the whole job.
Adults have rent, work, family, a hundred other obligations. Study time is the most expensive thing you own. A child's advantage is not a better brain. It is being allowed to get it wrong at no cost.
First: enough hours, without gaps
If you want to learn English seriously, you need at least seven to ten hours a week, sustained for at least six months, to move up a level.
Continuity is the crucial part. Study for a month and then stop, and the work before it is broken off. You will have to redo that same stage later. Stopping before you have reached the next level is where the waste happens.
Second: the right method
Among all the approaches on offer, you need somewhere that teaches well and a route that actually goes somewhere, so that every six months moves you up a clear level. A1 to A2 gets you basic conversation, then B1, then B2 and C1 for working fluently in English.
Eighteen months of consistent, well-directed work takes you from nothing to holding your own in an international workplace.