Hi Borderline, I think the reason you have had difficulty meeting someone or starting a new relationship over the past two years is.......you still have the ex in your life.
Even though you know it is a friendship in the back of your mind you still have a man so why look. Besides spending time with this guy takes away from your new life.
Granted lying to a good friend is not acceptable, but it is possible he was hiding the relationships so you would not be hurt.
You admitted when you saw him with a girl it really upset you. #1 You need to work on that, because you and he are friends, #2 Good reason for him hiding his other relationships.
He has every right as do you to spend time with others!
You seriously need to meet some other friends and possibly you will be attracted to one of them. Or join a fun class, art, etc, or some kind of community group!!!
I have felt some of the hurt you are feeling, so I can relate, but you are only hurting yourself. Try not to hold resentments toward this guy, you did admit he was caring and supportive. It is just time to move on!!!
good luck to you