It is natural that you would want to see your therapist. The opportunity to get everything off your chest and allow your therapist to not only share in your burden but to help guide you through and support you is very appealing. And it was a loss when you could not see her. It is important that you see it as a loss and allow yourself to feel down about it. But you will not go that low, because you have gone through this before and you have done just fine. It is just a matter of time moving forward until you see her again.
You are not out of control. Your feelings may make you feel that way, and it is a valid feeling. But you are in complete control of what you do and how you act. Allowing your feelings to dictate your actions is a way for you to cope. It is also a way for you to express your feelings, but not always in a healthy way. Letting go of your pain through cutting yourself is only a temporary measure. The pain always comes back. By letting go of that option and telling yourself you will not do it, you open up all the other options that do help. This may not seem appealing or helpful at first, but in the long run it is more beneficial to you.
Your goodbye letter in your journal was ok to write. You did not start off with the intention of hurting yourself. It seems that you may have just been expressing some feelings that you have. Wanting to die when you feel down is normal. You cannot pretend it doesn't cross your mind. But taking action is a whole other situation. And you were not taking action, only expressing thoughts.
Tell yourself that you have control because you do.