Honestly speaking, one of the greatest lessons I have learnt so far is to "Let go and let God".
What does it mean to "Let go"? It is to let go of your problems, troubles and everything that ever bothers you. To "let God" is to pray, to tell Lord your problems and believe in Him, believe that He will solve the problems for you. Once you let Him solve your problems, you will no longer be troubled. But first, you must have faith in Him.
For me, I have learnt to believe that God always have something planned ahead for me. And whatever that He have planned will definitely be the best. Sometimes I forget, but each time, He reminds me. I remember, when I was younger, during bible study, my mentor would tell us, don't worry because Lord will not let any of His believers suffer any form of pain. Look at the beautiful flowers, do they worry about not having pretty clothes to wear? NO! God have it planned for them already.
Sometimes, you would feel like you don't understand why you are going through certain obstacles in life, but do not fear, for Lord has everything planned for you.
There are always reasons when the window of opportunity is closing on you, you just can't see the reason now. One day, you will look back and realize that God has planned wonders for you.