How do I tell my parents I'm admitting myself to a psych ward?

If you want to admit yourself, you can do so at a mental hospital. Just explain to your parents that you are going through something so difficult that you can't do it on your own and you need constant surveillance to keep you healthy. Tell them you want things to be better for everyone in the family so you are making the decision to be proactive about your mental health. Good luck, and I hope everything works out for you.