When I was a child a, I remember that my parents had a very difficulty time to wakewaking me up oin time to drive me to school.

I could sleep uncountable hours oin a row without waking up in the middle of the night to use the bathroom.

I like the imagery of uncountable hours but I think it might sound a little better with a different angle. Like "I could sleep for ages..."

Likewise, I could also sleep for just three or four hours andbut still feel well enough in the morning to keep going with my life.

This isn't likewise really, it's kind of the opposite.

However, I guess that nothing lasts forever, though, since nowadays I have a much narrowerless flexibility on which a I can movefor how I need to sleep to feel well and healthy.

flexibility is not something that can be narrow usually, it's we usually say more or less. Maybe "be healthy and feel well" would also work.

As time goes by, I’m getting older and I need to carepay more attention to my sleeping hours.

In order to dTo that end, since it’s not that easy to fall I have difficulty falling asleep, I need to do more exercise and bare in mind that I cannot eat whatever I wanmore and watch what I eat before going to bed.

The sentence wasn't wrong but I'm trying to reel this in and be more consistent.

Therefore, I need to learn how to take a good care of myself to be in a good shape.

This sentence is unnecessary. Do you mean, something like "In summary" instead? It's not so much an if/then here.

Although once I’ve learned it, I feSince learning this, I have felt much better again and I’ve gained in physical resilience which makes me feel a bit younger.

Is this past tense or not? The verbs felt a little inconsistent

When I was a child aI remember thated my parents had a very difficult time toy wakeing me up on time to drive me to school.

I could sleep uncountable hours on a rowfor hours without waking up in the middle of the night to use the bathroom.

Likewise, I could sleep just three or four hours and feel well enough in the morning to keep going with my lifeday.

'Day' works better in this sentence.

However, I guess that nothing lasts forever, n. Nowadays I have a much narrower flexibility on which a I can move to feel well and healthycan no longer be as flexible with my sleep hours because I won't feel well.

I'm not too sure what this meant to say, so I corrected this the best I could.

As time goes by, I’m getting older and I need to care more for my sleeping hours.

In order to do that, since it’s not that easy to fall asleep, I need to do more exercise and beare in mind that I cannot eat whatever I want before going to bed.

Therefore, I need to learn how to take a good care of myself toand be in a good shape.

Although once I’ve learned it, Ithis, I'll feel much better again and I’ will have gained in physical resilience which will makes me feel a bit younger.

'This' feels like it fits the context of the text. I changed a few word tenses to talk about the future, because that's what I assumed you were writing about.

When I was a child a remember that my parents had a very difficult time to wakeing me up on time to drive meto get to school on time.

I could sleep uncountablemany hours oin a row without waking up in the middle of the night to use the bathroom.

However, I guess that nothing lasts forever, n. Nowadays, I have a much narrower flexibility orange of time in which a I can move tosleep and still feel well and healthy.

As time goes by, I’m getting older and I need to care more my sleeping hoursabout how many hours of sleep I get.

In order to do that, since it’s not that easy to fall sleep, I need to do more exercise and barekeep in mind that I cannot eat whatever I want before going to bed.

Therefore, I need to learn how to take a good care of myself to be in a good shape.

Although once I’ve leardoned it, I feel much better again, and I’ve gained in physical resilience which makes me feel a bit younger.