@principessa,
Well, you may have evidence. So go to therapy, and with an open mind. Give it a chance, at least for the sake of your child. Make an effort. This does not mean you stay together only for the child. But rather, be able to say to your child, if it comes to that, we did everything we could, but it still didn't work - not, that you just walked away without trying.
As for telling your family, maybe tell them a little. There's nothing wrong with saying that you are having some issues. You need not go into detail, at least not to start. Just, you are having issues and are talking to a professional, but right now you just want to work with the professional about it. You're sorry to be so mysterious but you want your family to know that things aren't fantastic, and you will explain later and really just need their support right now.