If you followed the instructions properly and took all the precautions, you should be good. Most suicides fail because of a poor choice of method, backing off at the last minute due to SI, failure to follow protocol, or being found early. If you did everything in your power to ensure a successful attempt, you should only have a slim chance of failure. Every method can fail, it's something that you have to accept, but worrying too much over it is pointless. I personally just write off the fear as SI.