Hi, I've walked in your shoes until very recently. I've just come from exactly the type of relationship you talk about. I'd been married for 20 years, but unhappily for around 10 of those years. No regular intimacy for many years and sex free for the last 3; rather embarrassingly I'd feel repulsed at the thought without drinking first, so I finally moved out of the bedroom. I have two amazing older teenage daughters, I thought I'd stayed in the marriage for their stability and to refrain from tearing their lives apart. I now realise I used this as an excuse to stay as I wasn't brave enough to be financially independent.
Anyway, long tedious story short, he was given attention by a colleague at work and started a relationship with her. My oldest daughter kicked him out when he 'mistakenly' sent me a photo of him and her (bastard lol) and I didn't fight it. It's been the most stressful 6 months ever as I had to find a rental properly quickly as my husband is military so we lived in married quarters, my income is hugely different from before so my family had to help, but I can just about pay my way by myself as I have an ok job. I've cried, and cried and cried some more with the fear of the unknown, but the reason I've joined Mumsnet today is specifically to tell you to not waste your life being with someone that you don't love. Yes it's hard, but if you are brave enough and in a financial position where you feel you can afford to leave your husband behind, your daughter will be upset but she'll also be fine.
My tears have now stopped, as have my children's, and the only feeling I have is that I wish I'd have done it far sooner. Even my children tell me I put up with him too long. I may be alone for ever (although I doubt it) but I was equally alone being in my marriage. What I have now is happiness as a singleton, with my girls to focus on, but with the added excitement that I may actually meet someone in the future that doesn't make me feel as though I'm dead from the neck down, or simply the live in cleaner and cook! Life is short, I'm 48 but I'm an eternal optimist. If you think you can be happier alone start planning. Good luck with whatever you decide to do though. It's such a huge and hard choice to make. X