This sounds like a typical young guy fantasy, and we get plenty of these kinds of questions on here. However, I will give you the benefit of the doubt, and treat your question as genuine.
You say she's blackmailing you, how can she blackmail you without implicating herself? As the forcing you to have sex, that is rubbish, a man cannot have sex against his will. Sexual arousal is necessary for a erection, and without an erection, sex is not possible. Therefore, you must have been sufficiently aroused to engage in an act of sex. By the very definition, it cannot be against your will. I'm not saying you didn't feel guilty afterwards, and maybe wished it hadn't happened, but to say that it was against your will is stretching our credulity a bit too far!!!
At 40 years of age, this lady is in her sexual prime, and not only that, she is conscious of the fact that her powers of attraction will soon start to fade, as age takes its toll on us all. Her husband being away just makes her more aware that she should be enjoying sex as much as possible while she still can. I'm not concerned in judging either of you, or making moral pronouncements; I leave that to others. She has already decided on what she thinks is acceptable behaviour, and the very fact that you responded positively when she seduced you is encouraging her to continue. You have to decide what you really want. If you don't want this to continue, then leave. Your 25 years of age for gods sake, not some silly teenager. You are perfectly capable of finding a place of your own to live if you don't want to be involved with this lady. Decide what you want, don't ask us. If you want what's on offer, take it if you don't, move out, it's as simple as that.
Remember one thing, if you get involved with someone else's wife, there is always a risk of being captured. Who knows what consequences there would be in those circumstances; so if you go ahead and make this a regular thing, keep in mind the risk involved.