There is no right time to CTB. It's something you decide according to your life and conditions. I wanted to try for my life before leaving, I did my best, now I think it's right time for me to leave. Other people may disagree, and argue that the right time to CTB as soon as possible, or maybe to do it impulsive. It's something you decide, no one can decide that for you.
As far as fear, I totally understand you. I'm also too scared to make such step. I'm scared to fail at it. However, I treat this fear like this: life is unpredictable, it can hurt me or reward me at any time. Any types of harm can happen to me at any time, but I won't live with fear thinking this is going to happen by 0.001%. The same thing goes with CTB, there is probability I will become a veg after a failing attempt, but I can't prevent that, I believe such harm will happen to me any how either by CTB or not. We're also destined to die at certain time (you may also believe in this), so if I'm destined to die with CTB, I'll be successful, otherwise, I won't.
You know, you need to take risks in life, you either take risks to live or die. Make sure you did all the attempts to live well, then death can be easier relatively.
And lastly, I'm not a pro-lifer or someone like that, however, I suggest you try to find solutions for your problems, maybe suicide isn't what you need in your life. Maybe you're looking for something different. Much love and tight hugs <3