First point I must make, after seeing that update that you are going to try to arrange an escape from this unhealthy arrangement, I would say go, don't look back, talk to her first, and point out a couple things; 1: You intend to end this to save what little self respect you have as a man, because an unclean woman who emotionally cheats on you is already 2 steps farther than any self respecting man should ever go down the hill of no standards, and trust me when I say that hill is STEEP. 2: To expect a relationship to work without a propper degree of honesty, is pure folly and the biggest part of the pattern we see in our horribly divorce ridden society, face it, it was over before you even decided to leave, it just would've taken time to rot all the way through leaving you emotionally clusterfucked and in no position to carry on a healthy life afterwards, because if you don't cut it off now the only thing you would have left is a life lived in denial and dissillusionment.
I know a lot of people might say having a partner for sex is worth a lot, but those people are almost 100% virgins who idolize sex in a way only the ill informed, or inexperienced can, sure sex can be great, but the sex you get when you don't respect yourself is a form of emotional torture that you can never fully enjoy, also women respect men who respect themselves and others, not men who say well whatever I'll fuck anything with a hole.
I have a decently active sex life, and have my fun, but I follow a set of rules that keep me from straying into the land of the "lost" as it were.
1: I will not under any circumstance, let anyone else shit on my life in a relationship, if a relationship is unhealthy for one, it only ever serves to reinforce unhealthy behaviour in the other, vise versa.
2: I will not change who I am, who I hang out with, or change any pre-existant plans for anyone, this is standard in life, as more romantic relationships fall apart on a yearly basis than solid friendships, and your bros. are who help you get your shit together again quick and reinforce you emotionally when a relationship breaks down, I may use social discression on who I ask to hang out with me and my girl as some of my guy buddies are... to put it mildly, afraid of women, and take out that fear through a mix of agressive speech patterns and unhealthy emotional agression bleedoff, but this is common sense.
3: I never give without a reciprocating return, if I cook one night she cooks the next, or orders pizza or something, food is food, that kind of thing, unless it is on birthdays, or if she's really struggling for food or something of that sort. If I buy her a B-Day present, it will not be something expensive, but it will be practical and thoughtful, and if she doesn't bother to return the favor or gets me a gift card for walmart or something retarded like that, I will be sure to remind her, that that is not how we do things, and that I do give hints when asked, if you have a hard time picking something out, just ask what kind of thing I might want, I will gladly give you hints that allow for room for suprises and I never buy anything I don't absolutely need within a couple weeks of my B-day.
4: I don't mind kidding around and playfully thrown taunts, but if a woman straight up insults my intelligence, or acts in such a way as to assure me that I'm a charity case or some shit, it's over, period, there are no second chances in this field, as there are MANY other women out there who aren't total assholes.
5: Cheating is an instant permaban from my life, no exceptions, if you cheat on me, I will sit you down for the last talk you will ever have with me, and tell you straight up, that it's over, I will not be returning calls, emails or text messages in any form, I will ignore your very existence, unless you buy something from a place where I work in which case I will serve you as a customer, and I will treat you with the kindness of a man who has no knowledge of who you are, you are dead to me.
6: I will never cheat on anyone ever for any reason, cheating is a sad weak man's approach to relational escapism, that said, I have unintentionally been the other man a couple times, and felt terrible for it. If I were ever in a position where feel that the relationship is worth putting at risk in such a manner, I have probably ended it months ago, in other words I don't cheat, I end relationships in a polite and direct fassion, and move the fuck on.
7: Last(that I can think of at the moment, as I live my life by a strict set of rules which are many) but certainly not least I avoid people who are too needy or clingy, I want a woman who wants me for me, needy women as with needy men are usually that way because they are in love with the concept of relational security, not the man/woman in question. I want someone who can be happy because they are happy, and want to share that with me, vise versa, I can be happy on my own or with friends, but with some people a connection develops where that happiness is indeed increased exponentially through a shared deepening of relationship, this is where love lies, it is not to be confused with lust or desperation, those are predominant in our society especially due to the way the ever present media influence pushes us to see life.
Cheers friend, may you be strong and love yourself, for you, and seek the happiness that is not tied down by the chains of expectation.