The no sex for 3 years, the sex once every month before the 3 years, the open relationship, her seeing another guy during that time, all this shit is some serious boundary-crossing taking place here.
I mean she was regularly getting pounded by some other dude... Like every day or every other day. Jesus man! How could you even stand to be cuckolded like that? She's not slept with you in years! Is she still seeing other people? I mean what the fuck? There is some serious dysfunction going on in your relationship. Why do you stay with her? You obviously provide for her other non-sexual (financial) needs, yet you have no problem when she goes out and gets fucked hard by one or more dudes. Note that if she's not sleeping with you during these 3 years, she's probably sleeping with other guys and in emotional relationships with these guys also (sex + emotion are tied together) Also you are probably in denial of all this and also you're seriously co-dependent to stay in this situation. Check her phone! Check her email! I mean accepting an "open-relationship" to begin with means you doomed your relationship. I've never seen "open-relationship" arrangements lead to stability long run. These "open-relationship" arrangements always spiral out of control. Why? Because sexual involvement is very very much tied to emotional involvement. Translation: She gets pounded hard enough by a stranger, she has 5 or more orgasms (that she thinks you could never give her), she starts to get emotionally attached to her fuck buddy and starts to idealize him (her internal voice is telling her "Oh God, if only he'll want a relationship with me.") The fuck buddy (male) does not give a crap about her, he's just using her for now. He may be fucking other women too. (that she does not know about) He may choose to ask her to be in a relationship with him and she'll drop you like a hot-potato, jump at the chance to be with him. How can you put yourself in that position? You're basically at the mercy of some stranger (a guy who fucks your SO regularly and for whom your SO already has feelings) who for now is using your SO sexually but who may choose to turn her into a permanent fixture in his life? Which would mean that you'd be permanently out of her life. I mean it's insane!
After reading "Stop Walking On Eggshells", I'd have the following response to your SO:
I'm not getting the emotional, sexual and psychological support that I need in this relationship. Because these 3 elements are absolutely necessary for me to continue going forward with the relationship, I've decided the best course of action for us is to go our separate ways.
This way, we will each be free to pursue and fulfill our individual relationship needs with other people.
I strongly believe that a mutually exclusive relationship based on respect, boundaries, trust and loving-kindness would meet my needs and this is not the case in our relationship.
The history of our relationship makes it impossible for me to continue going forward.