hi, i hope you are okay and i know i can understand to some extent the whole thinking process you go through because i also have boderline personality disorder, and i think reading from your post that you dont find comfort in your friend, you only find similarity. thats what is not common between you and your wife, and thats what you think "clicked" between you and this girl. but trust me you dont love her. you see yourself in her and you want to help her in order to help yourself. shes like a mirror to you. and being with a person who is the same manic/depressed as you will not lead you in to any stablity, the 24 years of marriage, the children, you will never attain from this girl. iall you wanted to do with her was to see yourself in her and try to amend the things in her that you find in yourself. but you are not a psychiatrist, or a psychologist. being a friend for someone to vent out is something else, but dont think going on being her life coach when you yourself need the sturdy balance andd the rock that is your wife.
you have cheated on her, and you should talk to her about it. you said you love her more, so tell her. 24 years of marriage deserves honesty. be brave tell her everything.
and for your girlfriend, i think you shopuld leave her. you cannot be a rock to someone when you already are crumbling..