Hi oggy, very sad when this happens,of course you need a life as well,you say you no longer love her, is this really true? only you can decide what will happen in the future, your wife needs you now, you also have your own needs sexually and emotionally,
would an affair solve your problems? I don't think it would, you would probably feel really guilty afterwards you have been with your wife a long time, as you say you won't walk out on her so all it seems to me is you are asking if it is ok to have this affair, I would say No it's not the way to go, you can sort yourself out if you need to.
Does your wife have good times as well as bad days? maybe a holiday in the sunshine would do you both good, you have a lot to think about, cos if you had made your mind up you would not be on here asking advice, hope things become clear for you soon, Take care, Ray