Regrets and Wishes

Expressing wishes (present regrets)

I wish I had more time.
I wish I was more disciplined

If only I had more time.
If only I was more disciplined.

I should have more time.
I should be more disciplined.

I wish/ If only  + subject + V(simple past)

I should + V (base form)


Expressing regrets (past regrets)

I wish I had trained more before the race.
If only I had studied more. (I failed the exam)
I should have studied more,

I wish / If only + subject + V (past perfect)

I should + V (present perfect)

Explanation
https://learnenglish.britishcouncil.org/intermediate-grammar/wish-and-if-only
https://learnenglish.britishcouncil.org/english-grammar-reference/wishes-and-hypotheses
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv301.shtml

Exercises
http://www.eflnet.com/grammar/wish.php
http://www.esl-lounge.com/student/grammar/4g7-wishes-and-regrets.php
https://www.esl-lounge.com/student/grammar/4g48-advanced-if-only-i-wish-tenses-multiple-choice.php