Being smart is like an exercise, if you keep practicing then you will be smart and if you stop the practice then smartness will be not be same as before.
Now let's learn the exercise to become smart?
Be silent, try to listen the surrounding sound, may be AC or water sound.
Listen to the thoughts you are thinking, listen to the instructions you are giving to yourself.
you may talk to yourself many times and instruct to be silent. (Don't you think, giving instruction once is not sufficient? why need to instruct multiple times? no confidence? trust me, mind knows what you want so it will help). To be mindful, we have to be in present, do whatever you are doing but with focus or mindfulness. If you enter in house and then you should be aware where you took off shoes and where you kept Umbrella or your coat, be aware.
Try to be quite, mind will play trick not to be quite, so if you understand those tricks and don't fall for those tricks, then you start becoming more and more smart. Don't force yourself to be quite, but understand the tricks and decide what is the best answer to respond (be honest with yourself, pray to god, you will definitely get help to learn. Key is to practice it regularly).
It's a practice, first be mindful and then observe your thoughts and then respond (here you take decision and you always know what is right and what is wrong. After some practice you will not fall for those tricks start becoming more and more silent.(remember these tricks are not many, most of the times few categories are acting as the main force behind those tricks, example tricks can be.. either driven by greed or anger or jealousy or laziness & few more...)
Please remember all thoughts are nothing but the chain of simple words, if you attach to them then you react, so learn to be detach and respond. Listen to those words and respond not react.