You should never open a relationship if both partners aren't enthusiastically for it. You did, despite not feeling comfortable with that. Now your options are the same as they were before, you either speak to your partner and go back to being monogamous or you speak to your partner and break up. Unfortunately you may simply be incompatible and as sad as it is, trying to hang onto your relationship as it is now is harming your physical and mental health. There is no trick to it, you are with the wrong person. Love just isn't enough for a healthy, fulfilling relationship.