She's happy around you b/c you take her mind off her marriage problems. You are a distraction from them and prevent her from working on them, please understand this important point. By being her emotional tampon, you are actively preventing her from working on the problems in her relationship. Why should she work on helping her husband to fill her needs when she has you? Or, possibly more interesting to you--your support actually helps make her marriage problems more tolerable. If you really think she should divorce him, you are actually preventing this from happening in a more timely manner.
No doubt she's filling some needs of yours too, which is why you are mutually attracted. Since you aren't a couple, its easy to see things as all sweet and light b/t you. You experience only the best of each other, in a very artificial circumstance. You are like a drug to each other, numbing yourselves from the more mundane problems of life. Like sorting out the inevitable problems that come with long marriage.
If you can screw up your willpower and wrap your brain around a way to just switch off and break contact with this gal, it would be best. If their marriage doesn't make it, you can deal with any feelings you may have then. But meantime, its just what Hayward said: lots of hurt, anger, discord and confusion for a long while.
If you can't do this, you will either end up in an affair or shredded. Probably both. After that you, or she, will still end up breaking it off. Instead of it being the clean break you can have now (by clean I don't mean w/o pain, btw), you will have damaged yourselves and any potential future relationship possibly irrevocably.
Take the direct path from A to B across no-man's land, if you can. Good luck.
Last edited by IndiReloaded; 23-12-09 at 02:56 PM.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh