I won't try to dissuade you, but I'll correct one thing you said and give true advice.
First of all, you can't say that you'll "never ever be happy" unless you can see the future. There's always hope that things will turn around for you. Instead, say: "I feel like I'll never be happy again." That's a true statement, but that doesn't mean that your feeling is true.
Secondly, if you do something impulsive, chances are that you'll end up surviving in an even worse state than you are in now. The only easily-accessible method that I can think of would be hanging, which takes thirty minutes to be surefire. If your parents surveil you like you've said before, they'd probably find, resuscitate, and save you, which would leave you with severe and irreversible health issues. Also, ordering things is risky because your family—even if you're certain that they will be preoccupied—always have a nonzero chance to intercept it or find it before you will. If that happens, they'll probably throw you away in a ward.