Medications can take several weeks to start working. But, if you're feeling so close to the edge, call your doctor. If you're seeing a family doc or a general practicioner it might be time to ask for a referral to a psychiatrist or a psychiatric nurse prescriber. You may feel worthless, but in reality you are worthy of seeing a specialist.
It can help during the darkest times if you can have someone you can trust to just sit with you - not to "watch out" for you, but just be near. Or even call every so often to just listen if you want to talk. I don't have anyone I can trust like that, so when I'm feeling suicidal I just clench up in a little ball and hold on to ride it out. Sometimes it help to blast my brain with loud music. The worst of it always does pass, but then I feel like I'm recovering from brain flu for a couple days.
No one has really explained "crisis" to me - as in "if you're in crisis, call the suicide hotline or go to the emergency room." But if you start getting urges that are difficult to control it's time to go to the ER. I never have, but that's the deal I've made with my psych nurse.
Hang in there. Post when you can or just read the board. Give yourself time.