A few weeks ago, I decided to resume my morning walks again. A few weeks ago, I decided to resume my morning walks.

Resume already implies you’ve done it before, therefore, you do not need to say again.

Lately, I wasn’thaven't been feeling too well and thought that it might be caused by me not doing exerciseing at all. Lately, I haven't been feeling too well and thought that it might be caused by me not exercising at all.

'doing exercise' is passable, but sounds odd in this sentence to me so this correction is just for sounding more natural.

Lately, I wasn’t feeling too well and thought that it might be caused by me not doingdue to my lack of exercise at all. Lately, I wasn’t feeling too well and thought that it might be due to my lack of exercise.

More sophisticated

It is not that I do not like doing exercise, quite the opposite; I love it. It is not that I do not like doing exercise, quite the opposite; I love it.

Beautiful grammar !

However, I had to stop doing exercise as a result of me having side effects forom a surgery a decade ago. However, I had to stop doing exercise as a result of me having side effects from a surgery a decade ago.

However, IOnce before, a decade ago, I also had to stop doing exercise as a result of me havinging because of the side effects forom a surgery a decade agoI had. Once before, a decade ago, I also had to stop exercising because of the side effects from a surgery I had.

Word order, better connectives. Connective 1: establishes past Connective 2: specifically details time.

Two days after my surgery, I realised that I was not able to raise my arms. Two days after my surgery, I realised that I was not able to raise my arms.

This is very formal; if this was your intent it is great. Otherwise, you could instead say “Two days after my surgery, I realised that I couldn’t raise my arms.”

I told my doctor so and began. As a result, my rehabilitation immediately. I told my doctor. As a result, my rehabilitation immediately.

Continuing the formal voice; and sophistication.

Not only was it for me very hard for me to physically to face such a terrible situation, but also mentally and physiologically. Not only was it very hard for me to physically face such a terrible situation, but also mentally and physiologically.

This word ordering sounds more natural and puts clearer emphasis on there being both a physical and an emotional impact.

Not only was it for me very hard physicallydid I find it very physically taxing/difficult to face such a terrible situation, but also mentally and physisychologically. Not only did I find it very physically taxing/difficult to face such a situation, but also mentally and psychologically.

However, my shoulders were very weak and wouldill be like that forever due to nerve damaged. However, my shoulders were very weak and will be like that forever due to nerve damage.

However, my shoulders were very weak and would be like thatcontinue as so, forever, due to irreparable nerve damaged. However, my shoulders were very weak and would continue as so, forever, due to irreparable nerve damaged.

More sophistication

And tThere is no solution for thatneed to put this in as we already know it is unfixable. There is no need to put this in as we already know it is unfixable.

I had to admiccept my new situation and learn to live in a different way. I had to accept my new situation and learn to live in a different way.

I had to admiccept my new situation and learn to live in a different way. I had to accept my new situation and learn to live in a different way.

Admit = to reveal fault/responsibility, OR to enter/be permitted into a certain place

I enrolled into a sports centre near my home and started swimming. I enrolled into a sports centre near my home and started swimming.

I knew what it meant to suffer doing exercise and also knew that whatever I did, I wasn’t going to risk further damageing my shoulders. I knew what it meant to suffer doing exercise and also knew that whatever I did, I wasn’t going to risk further damaging my shoulders.

But after somea few months, I had to give up. But after a few months, I had to give up.

But after someeveral moths, I had to give up. But after several moths, I had to give up.

There was no point in me doing exerciseing. There was no point in exercising.

Much more natural; always phrase it like this.

After several years, lLast week, I reenrolled again into another sports centre near my house with a swimming pool. Last week, I reenrolled into another sports centre near my house with a swimming pool.

The first day I went, I didn’t even remember how to swim. The first day I went, I didn’t even remember how to swim.

Very natural sentence good stuff

Although my goal isn’t to win an Olympic medal, at least, I’d like to be able to swim the swimming pool several times each day at least. Although my goal isn’t to win an Olympic medal, I’d like to be able to swim the swimming pool several times each day at least.