Feeling infatuation and loving someone are quire different things. You are mentioning that you want to feel like a teenager again and to feel in love all of the time. Those are both unrealistic and nothing last forever. What you feel now is the high of this possibility of a relationship and the residue of the infatuation. It is that which you are attracted to.
Unfortunately, any new feeling even that of the feeling of being in love wears off. What happens when people feel this way is also as a result of rise in feel good chemicals in the brain and hormones. Naturally people want to feel good and feel good more often than not.
Perhaps you may want to re examine this motivation in your most current situation and thing of the long term effects that this may have not only on you but those around you such as your friend and her family and your husband and the children.
Your feelings are in response to your thoughts and associations with them. Any person even therapists at times find themselves in a corner. That is human and natural.
You may be seeking something from him that you find lacking at the present time in your life whether it is attention, the novelty of a feeling and a situation, affection etc. As you sort those things out, you would arrive at a decision whatever that decision is.
There is nothing wrong in having coffee with someone you know.
It is possible that you may have discovered mutual interests and that in turn is intelectually stimulating to you.
There is no need to be anxious about this now. It will only make you more confused and uptight. Just see where he stands with you, sleep on the question after the meeting. There is no need to make any major decision right away.
Guilt is a dreadful feeling. You may want to explore the reason behind the feelings the two of you believe that you have.
Sometimes when people get comfortable in their relationship they get complacent and believe that with someone else things will be better. Of course, in all relationships all feelings change, mature and the feeling in love does not last forever.
Losing a friend to try and see where this new opportunity leads may not be healthy. Sometimes it is wiser to listen to logic than feelings.