Hello kid,
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You are 21 and you are just 21. I mean this is a time to do something, If you are searching for a Career after education then this is age to do something to step on a way to achieve something as we all consider major after 21 for jobs.
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Now, yeah that's common spending time with friends, having a fun day and time wasting etc. now we are in pandemic situation we all have nothing to do and that's ok. That's seriously ok. But make a thing clear, once the pandemic situation ends, our life starts running, once we get our routine life, you be how you were or you Change how you want to be.
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Have fun but even think of your life, what you want to do, what you want to be, how you want your life. Plan things and keep working for it. Where are you and where you want to be are the only 2 things you must see if you want to push yourself.
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As you said you want to prepare yourself for examination, make a plan of what you want to do, what you will do, how to achieve, how to prepare, books you need for preparation, yeah now it's time to be serious. So, you had fun till 21 years and now it's time to be serious. Remember your life gives you want you worked for, so work hard so that it can give you best.
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First change your routine you will find results automatically