Hi Marleyfan,
Welcome aboard.
I am sorry that you are going through this. I understand how difficult it is to feel that you have to be the one who has to keep it all together. It can feel overwhelming at times.
You have the right idea when you are feeling this way. Writing or talking about your emotions and feelings really helps when you are overwhelmed. Sharing your story really helps. You will find that there are many people here that are going through similar situations.
I understand that you do not want to lie to her. There are ways to communicate with a person with BPD (pwBPD) without having to walk on eggshells or lying. Communication techniques have worked wonders for my relationship. Take a look at this article.
Communication tools (SET, PUVAS, DEARMAN)What happened that caused your wife to go back in the hospital?