Hi - I know you feel terrible but the good news is that you are not actually trapped. I know it feels like it with fibre of your being but there are solutions and positive ways forward.
Ask yourself, what would telling them do for you? Yes, they would know, and to a degree they would know how you feel about things. But then what? Are you going to start transitioning right away? If not are you still going to change how you present in front of them, ask them to use a different name and pronouns?
I don't know where you are in your transition, or if you are pretty much in the formative stages. Are you talking with a counsellor, it seems that you need to find a way to deal with your own feelings around your gender identity before you lay it on the others. If you tell them anything before you really know what you expect to do about it yourself then I suspect things won't go well. This is just a personal observation, but it seems to me that having some sense of purpose and a plan moving forward gives you much more to stand on if others are unsupportive. It may also help to tell them one at a time rather than all at once - one on one is daunting enough, but a whole bunch of people can be very unnerving. It's no wonder you didn't feel you could tell them, so please don't beat yourself up about it.
Personally, I'm not a fan of letters. I've heard too many stories on this forum about people leaving out letters and then, when no one talks to them about it (which seems to be common) they start to feel very uncertain and unsure about what is going on. But you have to do what you feel you need to. I don't believe you are "chickening out" if you chose not to tell them just yet - especially if it gives you the opportunity to get some clarity for yourself first.
BTW, I know you probably weren't serious about "ripping it off" in the shower, but seriously, don't do that. It would make things considerably worse for you as - should you decide to get GRS later down the road - you'll really need every part of that part of your anatomy for the process...