Cortisol is a stress hormone, so can be lowered through relaxation methods - breathing exercises, yoga, meditation etc
However, being in a new relationship is bound to stir up lots of emotions, fears and anxiety if you have been in a very abusive relationship before that. Well done for getting out of it, however it will no doubt have left its mark on you psychologically. Did you have any kind of therapy after you came out of it? You can access CBT type therapy through the NHS Talking Therapies, which can be helpful for dealing with anxiety. Good to hear that the new man in your life is being supportive though.
Regardless of whether or not you choose to take any medication, I think it would be helpful to talk through your thoughts and experiences with someone trained in psychological therapy, as medication does not deal with the underlying issues and as you say, can cause issues of its own.