Driving my anxiety crazy

We all look through rose tinted glasses concerning our 'first loves" In reality you can never recapture those first flushes of love or that feeling . 

Time stands still for no man and nor does that feeling from all those years ago; it is impossible as you have grown in to an adult. You were both young and didn't know any different.

You also broke up for a reason.

As regarding those feelings of no longer getting butterflies  within your new relationship that is normal, relationships have to be worked on. All relationships will eventually lose those sparks /butterflies in time as we settle in to them. It is up to us as individuals to keep that spark alight.

The excitement is probably caused as a result of doing something that you know you really shouldn't. Once the novelty wears off it will be no different from what you have got now

As the ex wife [over 30 yrs marriage]my now ex did that and he probably felt the same as you; excited, couldn't believe that he had 2 women at the same time. However, in the end guilt got the better of him and believe me he has lost everything when he was found out.

Invest time and effort in the man who you say gives you a perfectly good life

Have date days/nights. Do silly childish things. Inject romance.

If you are prepared to lose everything based on childhood memories then do the honest thing before you hurt your partner; tell him please don't do it behind his back.

I wish you well.

It is always easy to just remember the good times of a past relationship and forget the bad. This relationship ended for a good reason and you were happy with your decision until he contacted you out of the blue. Is it possible that his interest is sparked by the fact that you are in another settled relationship and it may abate when he succeeds in breaking that up. We all remember our first love fondly but that's what they are the"first", when we are young and inexperienced. In my experience you can never go back, you are older now and want different things than when you were 14. My advice would be to cut off contact treasure your memories and look to the future. I hope what ever you decide that all works out well for you and you are happy.