I'm glad your therapy went well today. It's also good that you showed her the collage and let her know that you do not enjoy doing them. Not every therapeutic exercise helps everyone.
One step at a time with your letter I think is the best option. It sounds like you are getting warmed up, letting your therapist know a bit at a time about your letter. That is a good way to go with it.
You are not going to go crazy. I can definitely say that. OCD does not make people crazy. It is an anxiety related disorder that can make you feel crazy, but that does not equal being crazy. Anxiety disorders often make people feel crazy because of the out of control feeling that is part of the symptoms. Rituals help make you feel more in control. OCD is helped by medication and if you feel you need an increase, your doctor can help. If you feel a short stay in the hospital might help you, then it's worth a try. But you also may need a small adjustment in your medicine. Your body can become used to the medication and it becomes less effective so a small adjustment might be needed to reduce your symptoms.