The mental calculation you are making assumes that you cannot get yet more unhappy in the current relationship. You assume that you will continue being as unhappy as you currently are with certainty. You compare it with potential happiness that comes with uncertainty and wonder if the known evil with certainty might be better than a potential new evil that will come your way should the uncertainty resolves not in favor of becoming happy but the other way around. Other posters have already pointed out other flaws in your calculus and I am writing to add that the assumption of "known evil with certainty" is incorrect. There is a high likelihood that as time goes by, you will become more and more frustrated with an unhappy in your current relationship. And by comparison that will make taking a risk by leaving appear more attractive.