I had to study more in high school, had to think about my job suited for me when I was in university. I had to study more in high school, had to think about my job suited for me when I was in university.

Did you regret having to study more in high school? If so, "I regret that I had to study more in high school and think about how my job would suit me in university."

I had toshould have studyied more in high school, had to thinkshould have thought about mywhich job was suited forto me when I was in university. I should have studied more in high school, should have thought about which job was suited to me when I was in university.

"had to" here implies that someone or something was making you do this. In the context of wishing you had done something differently it's better to use "should have". It's a similar nuance to ～ばよかった Eg: もっと勉強すればよかった is similar to "I should have studied more"

However, even if I reallyhave regrets about my past, nothing change at this time or mys in the present or future. However, even if I have regrets about my past, nothing changes in the present or future.

However even if I really regret about my past, nothing change at this time or mygret my past, I cannot change it now, and I can't predict the future. However even if I regret my past, I cannot change it now, and I can't predict the future.

Not entirely sure what you were trying to say here but this is my best guess Regret is not often used with "really". You will sometimes here "deeply regret" though, so if you'd like to intensify it I would recommend using that The "I can't predict the future" part is mostly a guess becuase saying you can't change the future would contradict your next sentence

All I shouldcan do is making my decision atry to make the correct decisione. All I can do is try to make the correct decision.

All I should do is makinge my decision a correct one. All I should do is make my decision a correct one.

This sentence, even with my correction, feels a little odd to me. I would reword it to "All I can do is try to make the best decisions in the moment."