Take a moment and think about everything you are grateful for.
It's very easy to get sad over something you don't have right now. It's even easier to blame yourself for every thing that's wrong with your life. But what will that give you, except negativity? Whoever we meet are whining about the smallest of the things they don't have or complaining about those things which they have. That's how we connect nowadays right? By negativity. Isn't something seriously wrong  us teens nowadays! We act as if we have already lived the whole of our life and are out giving advices thinking we know everything. Everything is in the tips of our fingers nowadays, then why the dissatisfaction? I guess that is the main reason we are dissatisfied. Unless we work hard for it day and night, we will not feel accomplished. It is only when we put our sweat in it, that we will begin to value what we do. Because only then the victory will 'belong' to us.
There is no fun in having all the money in the world, unless you are happy in life. That inner peace is very essential. It is necessary to give time to your family and friends and not get too much carried away by the fear of your future. The fear will never end, you will always aspire for more, which is good. But your time with your loved ones may or may not return. So make every moment with them count. Invest your time with them. There is no point in regretting later. Remember it is not a 'waste' of time in doing something which gives you happiness. We do not know what life has got in store for each of us. One day it will be a bed of roses and with the blink of an eye, it can change into thorns.
It is okay if you have a small circle of loved ones. But that doesn't mean you shouldn't socialize. Know your place in everyone's life and act accordingly. Try to talk to as many people as you can, it will help you learn atleast something. Be kind and avoid negativity to the extent possible.
Do not feel inferior seeing others. You may see others laughing and having a great time, but you do not know what runs within them. There is no point of comparing them with yourself. You may be of the same age group, or pursuing the same career, but understand that both stand completely on different tracks in your lives. The finishing lines for both are completely different. Do not feel discouraged if the other has reached theirs. That is not YOUR goal.  Yours may be far, and it may be more fruitful. Patience and perseverance will lead you there too. Just trust yourself and DO NOT put yourself down. That's where optimism plays a huge part. If you are very depressed, and cannot even think about being happy, then try faking positivity. Smile more. Try to positively look at every action of yours. The moment you feel negative, distract yourself. It will be hard in the beginning, but be consistent. You will eventually see that being positive will bring out the best version of yourself. And it's that version of you that people will love the most!

Keep smiling, even when the clouds are dark. The sun will shine soon.