He is fucking someone, sorry to tell you this so baldly. Of course he doesn't have feelings for you, he is infatuated with the heady new of someone who is on her best behaviour. You can't compete, so don't try.
The wanting to be friends etc. is so that he doesn't have to feel so guilty about what he is doing. It also keeps you as Plan B because he wants to do exactly what he is doing, but it not cost him anything in terms of losing his family.
Sorry to be so blunt, but I am preparing you. Look after yourself. Now is the time to start living life for YOU. This crisis is the time that you attend to what you need to attend to (weight, health, getting independent, getting your own friends and your own life, do you know what is going on with the finances, bills, mortgage, insurance etc?).
You are currently numb and in shock, then you will be in so much pain you want to die. But you WILL get over this. You WILL rise to the crisis. I promise you. Good luck, and we are here for you.