Yesterday

Yesterday I went to school and I studied a lot. (This is fine or since you have alaready mentioned yourself, I, you can remove the second I.) Yesterday I went to school and studied a lot.

In the afternoon, I went to the gym to have training. (This is fine or you can place a comma after afternoon for emphasis and easier reading. Also we would tend to say, went for instead of went to have.) In the afternoon, I went to the gym for training.

I enjoyed a lot myself but I am really tired. (We cannot enjoy a lot rather, we enjoy something a lot. Also you were talking about yesterday so the last sentence really needs a time reference for the past. I am indicates now.) I enjoyed myself a lot but I got really tired.

yesterday

Yesterday I went to school and I studied a lot. In the afternoon I went to the gym to do some training. I enjoyed myself a lot but I am really tired.

yesterday

Yesterday I went to school and I studied a lot. In the afternoon, I went to the gym for training. I enjoyed myself a lot, but I am really tired.