It is very natural for you to have good days and bad days as you work towards feeling better. Some days are going to be difficult, because you are trying to cope with a particularly overwhelming trauma. And no one starts on the path of working through such a trauma without taking a few steps back at least every once in a while.
You are doing the right thing by facing your feelings. You wrote the letter, read it to your therapist and triumphed over what happened to you. But sometimes when you feel good about a step forward, you get reminded that you are still struggling. Accepting this as natural will help you work with it, rather than against it.
Also, I can tell that you are beginning to heal. You are turning to your therapist and to me asking for help when you feel you need it. That is a very strong sign of healthy feelings. You are more open to support and are not responding by hurting yourself first. That is quite a change and one you can be proud of.
Keep in mind that you are not at fault for anything that happened to you. Yes, you did lose time in your life dealing with the trauma. But most everyone has things that happen to them that are detours in life. Some have physical illness, some lose entire childhoods to abuse, some are addicted to drugs. Recovery from these things is long and takes much effort. But don't forget that while you are recovering, you are learning, growing as a person and gaining a lot. I do not believe people suffer without a reason. And your reason may not be clear right now, but you will see it eventually.
Hang in there. Try some of the techniques you have used in the past to help yourself feel better. Relaxation techniques are helpful. They give you focus and also help you calm down. Plan something special to help yourself. Look forward to it. Call someone who understands. Reach out tomorrow to get support either from family, friends or your therapist, if possible. You can also write me. I will be here most of tomorrow. Even if you just let me know how you are doing, it may help.