On this page, you can learn Japanese grammar to talk about the situation you don’t want to have.
You need Verb てForm for this sentence pattern. Click here if you don’t know it.
I regret + sentence
verb てform + しまう
This is used when you want to imply that you don’t want that happen.
I lost my phone.
People understand your regret with this sentence. It is clear that you did not want to lose it but you did.
You can say けいたいをなくしました。which also means ‘I lost my phone.’ but it sounds like you don’t care about it.
Summer holidays have finished.
The sentence implies that you really did not want it to finish. You want to have more holidays. You can say 夏休みがおわりました。 which also means ‘Summer holidays have finished.’ but it sounds no regret. You are ready for a new school term.