I had a my plan when I was still in university, but that didn't go well. I had a plan when I was still in university, but that didn't go well.

"my" should not be used here

I had a my plan when I was still in the university, but that didn't go well. I had my plan when I was still in the university, but that didn't go well.

I had a my plan when I was still in university, but that didn't go well. I had a plan when I was still in university, but that didn't go well.

Because you wrote "I" it is already assumed that it's your plan

I had a my plan when I was still in university, but that didn't go well. I had a plan when I was still in university, but that didn't go well.

I resigned from my job seven years ago because of my sickness, after which I recovered. I resigned from my job seven years ago because of my sickness, after which I recovered.

「病気になって仕事をやめた。そして治った。」という文章ですか？

I resigned from my job seven years ago because of myI got sick, after I recovered I resigned from my job seven years ago because I got sick, after I recovered

I resigned my job seven years ago because of myI was sick, afterbut I recovered. I resigned my job seven years ago because of I was sick, but I recovered.

Using "but" makes a more clear comparison between the before and after.

I resigned from my job seven years ago because of my sick, after I recovereddue to my sickness. I resigned from my job seven years ago due to my sickness.

After I recovered, I studied for qualifications and failed. After I recovered, I studied for qualifications and failed.

I am back in work now and have been studying English foer my future. I am back in work now and have been studying English for my future.

It would be good to specify that you have returned to work, since you mentioned that you resigned from your job earlier

I now work now and have been studying English foeto better my future. I now work and have been studying English to better my future.

This is a bit more natural sounding.

I'm working now and have been studying English foer my future. I'm working now and have been studying English for my future.

Using "working" makes it clear that it's a continuous action.

I work now and have been studying English foer my future. I work now and have been studying English for my future.