No one is going to advise anything other than cutting off the OM immediately, because it is the only sensible thing to do. If you feel the need to tell him anything, you could tell him that your family is your priority and you need him out of the picture so you can focus on it.
But stopping contact with OM is only part of it. Even if you did that this minute, the issues that have caused you to pursue this relationship will still be there, and you could find yourself dallying with someone else in time.
Your posts sound to me as though you are thinking about this situation (and perhaps your life more generally) in a very binary way: safe vs exciting; rollercoaster vs rut; passion vs complacency.
Relationships do change over time and the initial rush fades, but life does not have to be either madly high risk or entirely grey.
Can I suggest you shake things up a bit, particularly if as you say, you are working up to cutting contact with OM? By that I mean you don't have to do anything as you have always done it. Free yourself to change anything and everything. Put OM on the back burner (you needn't delete him, but don't contact him) and really concentrate on your family life. You will be amazed how refreshing it can feel to make changes and use the ability you have to make choices in a positive way. Start talking about hopes and dreams with your DH, with your children. Seduce your DH instead of the OM.
Your mentions of two things struck a chord; past bad relationships, and living in a fantasy world. Ditto. And I found that the bad relationships (sneaking around, will he won't he? is it on or is it over? shall we see each other, is he angry, is he cheating? etc.) had effectively trained me to associate 'love' with an extreme adrenaline rush that was more akin to anxiety. If it wasn't a cortisol overload, it wasn't real. The sort of sweet kindness of nice blokes who really liked me made me uneasy. I know now I had low self-esteem and was terrified of real intimacy.
And the fantasy-also tied in with low self-esteem. I had a completely unrealistic picture of the sort of amazing heroine I had to be in order to have worth. Black and white thinking again- you're either Susan Sonntag meets Wonderwoman meets Rosa Parkes meets Jessica Ennis-Hill, or you're nobody. If the men you meet don't sweep you off your feet in some impulsive way, you must be rubbish. Worthless.
If this rings any bells for you, then please seriously consider seeing a therapist to untangle this. You've got a lot to lose if you carry on down your current path.