That you were born alone and you will die alone. Try living a meaningful life as much as possible. Have goals and work on them daily if you can.
You won't be young forever. Clearly your biological clock is ticking every day. You get those small reminders of your mortality.
You may have over 10 people who you consider as friends but only 1 or 2 will be there for you during your hard times. Be even thankful if you have 1.
That you are not that important no matter who you are. Everyone has their personal life and is engrossed in their own problems. Don't think too much what people will say because in the first place they are not thinking about you.
What is in your mind is an exact reflection of what is happening in your life. You wonder why you keep making the same mistake over and over again or why you keep failing at something you are good at. What you need to do is go through your beliefs and self talk and eliminate those thoughts that are not working for you. You do this by replacing old thoughts with new ones. I call it displacement
That you will have depressive periods in your life. Not everything will work out exactly as you want. Rule of thumb is don't be too attached to the outcome of every event.
That luck plays a big factor in becoming successful or a millionaire. I know People who it was their first time trading financial markets and they just went all in on one trade which got them to the top 10% of the world.
To become a professional sport athlete does require you to start early and to mostly focus your life on just that one sport. This people have put over 10,000 hours in what they do to be that good. That was according to malcom gladwell. I agree with him because half hearted athletes never make it big. Do you know any world class teniss player,nba or football that has a day job?
That money is important. Most couples break up because of money issues where one wants to spend and the other is not working or not earning enough. Make sure you learn new skills that will help you make enough for how you want to live. I would advise you to start saving early in life to be able to have enough for the future.
Happiness is found in the pursue of a worthy endeavor. Material things won't contribute much to your level of happiness. Feel free to add yours, we learn everyday

Person Wey dey shit no dey laugh.