The saying is that running is a natural movement for the humans, but for me running for more than 3 minutes was just impossible.

There is a saying that running is a natural movement for humans, but running for more than 3 minutes was impossible for me.

My body felt heavy, my mind tired.

My body felt heavy, and my mind was tired.

But now I'm seeing the improvement.

Now, I´m seeing improvement.

For me it's all a mental.

all mental.

For some reason my mind always want to abandon.

For some reason, my mind always wants to quit.

Sometimes I'm sure my body can do more, but my mind is just so obsessed with being relax.

with relaxing.

I hope I can learn how to master it, in order to run for more time.

I hope I can learn how to master it to run longer.

But now I'm seeing the improvement.

For me it's all a mental.

For some reason my mind always wants to abandonquit.

"Abandon" needs an object e.g. my mind wants to abandon the attempt. In any case, I think "quit" is the most natural verb for running, either quitting this run or quitting running entirely.

So, I control it as long as I can.

Sometimes I'm sure my body can do more, but my mind is just so obsessed with being relaxing.

"being relaxed" would have been correct for your chosen phrasing, but "relaxing" sounds more natural.