Study. Study. Study
I am not sure if the phrase “learning every thing by hearth” in French is the same as the English phrase “know by heart”. I figure that your question just has a typo. You will need to learn to love learning.
Admittedly, I only know American military pilot experiences second-hand. But, I have flown with military pilots on the civilian side. What will be common in aviation in general is that you need to love to learn. And some of that knowledge, you need to learn to know by heart. Just remember, it’s not impossible. Millions of people have done it before you. You, also can do it.
Almost everything you do in an aircraft has a checklist. That checklist combined with the rest of the AFM and supplements is your airplane bible for the moment. There is very little you should be doing that is not found in a checklist. I often tell new pilots to go through the full checklist even when they fly the simulator or chair fly.
Most of the checklist is a “Do then Verify” list. That means you will have to memorize a lot of procedures so that you can perform the memory items when speed is of the essence. You use the checklist to make sure you have done everything to standard and not missed anything.
This is just one example of knowing it by heart. Committing things to memory and muscle memory. Practicing and rehearsing and practicing and rehearsing. And when you are done practicing and rehearsing, practice and rehearse some more. A lot of civilian pilots have a background with that type of discipline. People who have excelled at a musical instrument comes to mind.
You can teach yourself that discipline by practicing it everyday in whatever you do. Build great study habits and stick to them. Practice deliberate and precise driving. Refine your fundamental skills and techniques in whatever sport you enjoy. Or, figure out a way to start flying now. Be purposeful and diligent in even the little things in life. No, especially the little things in life.
Hope this helps.