With previous attempts, there's very little chance that they don't involuntarily commit you. I don't know your method, but with most it's pretty obvious that they were attempts, it would be pointless to use the ol' "I feel directly into this noose", or "I really have no idea how this obscure chemical/one hundred pills found their way into my stomach"... you get the idea.
However, if you are calm, a good liar and respond to them in a submissive way, you can really shorten your stay and have a much easier time there. And depending on the psychiatrist, you may only stay until whatever wounds you end up with are healed. They can't lock you forever, you'll eventually be free one way or the other.
I really feel you, being committed after failing is one of my greatest fears. But don't think too much about the failing aspect, most methods also have a much higher success chance, so just try to take comfort into that and make all the preparations that you need.