Firstly I will say that you are not crazy. Older people always look at children and wish life was as easy as kids see it. For most people there is normally a slow transition into the 'real' and (sometimes) harsh world, however some find it out very quickly and it is a distressing and unpleasant realisation. Life is hard work and has a lot of 'ups and downs'. The key point of that last statement is that it includes the word ‘ups’. The world sometimes gets overbearing, yet it uncontrollably swings around in the other direction and will bring you such joy!
A lack of trust in others is not uncommon and people do mask this with a front. You have worked out from experience that some people should not be trusted, however don't forget to realise that not everyone is that way inclined. At present your lack of distrust could have stemmed from parents, embarrassment, tormenting, etc and it is hard to deal with, however it could eventually be character building and will become noticeable it the way you treat others and your methods of conversation. This is a good thing as you now realise ‘what not to do’ and you will treat others with the respect they deserve. It will be hard to accomplish this realisation however if you sit around your house and get ‘stir crazy’ this will further feeling of being down on yourself and your lack of confidence. When you look in the mirror and see yourself as ‘ugly’, you have just forgotten that there is always someone uglier than you because EVERYONE has a different idea of what is ugly. If it is people saying that you are ugly and making you feel the way you do, they will no doubt have issues of their own. The last statement is not very helpful to you, but hopefully you realise that you will have to work out and draw a line (always reassess this line however!) as to how much trust you can put into different people and then enjoy what’s left (and eventually, if possible, learn to take everything else with a pinch of salt). Don't forget that generally people will be great company and be loyal friends.
I have known friends who self harm and it provided a ‘release’ and a distraction from their depression. It is a very extreme method which will further lower your self-esteem and provide a permanent reminder of how much you hate yourself, therefore I would advise strongly against it. If it is a distraction that you are after, take a step backwards and look at what you actually find interesting and what gives you motivation and happiness...start trying out different fun things! An easy example would be to leave the house and explore the world in a different way and this will help you to stop getting ‘stir crazy’ indoors and could provide more inspiration than the internet (I’m guessing it’s the summer holidays too?).
Overall you are young and still have a lifetime ahead of you that will be packed with such great memories that you will miss being young. Time will change everything and you will become someone with many devoted and loyal friends/family and you will hopefully be able to offer wisdom to others experiencing similar problems and make a difference to the world. It’s a ridiculous idea to even think about ending your life and it is seen as a cowards way out, and somewhere inside you, you must find that it is NOT the answer and know that there are people out there you will bring such meaning to their life...I promise.