Things that I learnt from my mistakes:-

Never ever choose a miser as your roommate.
The one who waves at you and does small talk may not necessarily be a friend. There is a big difference between a colleague and a friend.
Maintain a budget. You won’t turn bankrupt, 10 days prior to the end of month.
Don’t make future promises on the basis of your present circumstances.
The people who promised everlasting togetherness today may disappear tomorrow in thin air. So, spend time alone and make peace with solitude. One day, it might be the only option left to you.