I agree with A Red Panda: individual therapy.

Emotional blackmail occurs when there is a threat of illness, self-harm, suicide if one leaves. It sounds like your friend might not intentionally being using emotional blackmail to I keep you, but it is still happening. You are afraid to leave because of what might occur in her life.

In individual therapy you can work on your seeming need to rescue others rather than allowing them to be responsible for themselves.

If you prefer your wife as you say, then you CAN let go of the other relationship. If you continue to choose not to, then ask an individual therapist for help in making the choice that you CAN make and want to make.