You need professional help (which doesn't have to be "threatened"). Please rock up at the Emergency Department and spend a week or so being treated for depression and self harm properly. When a 13 yrs says "I have suicidal thoughts every day and night and they are getting stronger so I don't even trust myself to be alone sometimes" you are crying out for help.
Your brain will keep having these thoughts. No wonder you "don't remember what it feels like to be truly happy". The fake smile is good. At least it shows you can still socialise somewhat. A real smile would be better though.
Even if the hospital don't admit you at least they can do a full initial assessment and give you some community contacts. Just take someone you trust with you not someone that will huff and puff and say, after a 2 hr wait, "you know we're missing the State of Origin ?". Although having a fake smile when NSW lose again might be of some benefit. Go the Blues ! Er, well there's always next year.
You have your whole teenage life to look forward to - zits, puberty, boyfriends' first sloppy kiss behind the bike sheds, homework, tests, that cute boy in the Geography class that wants to ask you out but is scared to, Auntie Matilda squeezing you in her ample bossom when you graduate and finish HSC, University selection or Employers waiting for your cv and, most importantly, 2 years to recover from the wedding of the century - Justin Beiber & Marge Simpson. You get my drift ? Self harm is looking inside yourself and most of life is about going outside the box and doing stuff that is fun or safe.
Being "afraid to tell anyone or ask for help" is OK. But you can always email, text, write messages in the sand at Bondi Beach, etc. You are actually in need of some help, afraid or not. The longer you leave it the more afraid you might be. Just chill. Slip it into a family conversation and no one will even notice:
Dad: "Bloody government want to start charging fees for public school"
Shay: "I've started hurting myself and I feel like I want to die"
Mum: "Oh, that's another thing, the lawn man is coming on Friday this week - can you organise for one of the neighbour's to pay him, thank you darling ?"
Shay: "So I need someone to take me to hospital after dinner"
Mum: "Gavin, you take her - I need someone to grab some milk from the supermarket as well. There's a Woolworths by the hospital, it's on the way."
Dad: "Hospital - is it serious ?"
Shay: "Yes, I've been afraid to tell anyone"
Mum: 'Shay for goodness sake - remember the time I got caught in the toilet at the shopping centre - that was pretty serious and I told you all about that. And get 4 onions for the mince I'm cooking at the weekend. Gotta do that dish that Uncle Harry likes so much for his 60th birthday. God, I remember the day we all went to see 101 Dalmations and he tried to sneak his puppy into the cinema..........."
Chances are there is enough drama to absorb your drama. If you have to get hospital treatment a 2nd or 3rd time the attention will lessen even more. You can't expect your family to accept it at first, but they'll come around and support you in some fashion. Because that's their job. Now, go wait in the car while I find the car keys.
PS I have 3 kids 18, 20 and 21 and compose jazz/kids songs. There is some humour in the response but you know I'm am concerned and serious at heart. The moderators wouldn't update this if they thought it would offend you in anyway. Good Luck and welcome to BB.