During/For the Last 3 months I have exercised a lot. During/For the Last 3 months I have exercised a lot.

You can say either "during" or "for"

As result I have managed to reduclose some weight. As result I have managed to lose some weight.

Or you can say "I managed" We say lose for weight loss in a person. You might say reduced weight in an object

But l do feel tired and is hard for to push furthermyself to do more exercise. But l do feel tired and is hard for to push myself to do more exercise.

Or: hard for to me to do more exercise.

The other option I can try is to reduce how much I eat per day. The other option I can try is to reduce how much I eat per day.

Actually I doid try that method a couple of years ago. Actually I did try that method a couple of years ago.

Good use of "couple" rather than "two". It will always be "a couple of" rather than just "couple"

The result was good I losst so much weight. The result was good I lost so much weight.

But I was overly do thaoverdid it and gettingot really sick. But I overdid it and got really sick.

I hope this time I will get the right balance. I hope this time I will get the right balance.