I have to say that I'm very regret for not putting much effort on learning when i was in school.
I would change this a bit:
I have to say that I deeply regret not putting much effort into learning when I was at school.
This is because 'regret' isn't an adjective. You could use 'regretful', but that doesn't sound as natural to me. Welcome to the forum. I will have a look at the rest of the paragraph in a minute when I have time.
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