HELP! I need adult advice!?

I am 40. I have been seeing "John" exclusively for about 2 months now. I met "David" about 10 yrs ago through mutual interests (we frequent the same church, etc.)

Last night I went out to dinner with David (as a friend) and we really hit it off. He even bought me roses. (nothing physicaI happened) I plan on telling David about John today, but am sure I will lose John if I tell him about David. I am so confused and this is making me sick because I don't want to hurt John. This has never happened to me before. What on earth should I do? Adults only please.