I'm glad to hear you are feeling as if you have a little more control over your emotions.
Ultimately, you are the only one who can know for sure what is the best thing to do in any situation. Every poster on here can only speak from their own experience, and that is never going to be exactly the same as yours.
However, what does come across in your post is that your emotional energy is being spent on OM. You talk about your relationship with him with more subtlety and insight than your relationship with DP. The need to get somewhere honest with him, the things that were preying on your mind.
From the start of this thread, the big imbalance has been exactly this - where your investment of time, emotion and energy is going.
Whether or not OM is a decent guy only matters if your emotional connection with him is going to continue.
You have been agonising over OM and putting more time and energy into your connection with him. I'm not talking about this being right or wrong in an ethical sense. But the simple fact is that if you put this much effort into your relationship with OM, it strengthens your connection with him. And you get further from your DP in the process.
I think it is good that you don't feel so awful any more about what happened. But you say that it will make it easier for you to move forwards now. And I am sceptical. As I said above, this is because of MY experience, and we are different people. But there it is.
Of course the pain has lessened. You have been in touch with him. You have had reassurance and a declaration of care and affection. That's bound to ease the pain.
If you really believe that you can now move forward, great - go for it. But a few sentences further on you say "I don't think we'll meet again, other than to say goodbye." That isn't moving forward, Mabel It is holding on to the past.
How many times can you say goodbye? You have already said it. It has already been done. You were saying this close to the start of the thread - about needing to see him one last time and have a proper goodbye. You are going to step on to the same wheel again.
I don't want to sound hard or unfeeling. I can imagine the pull of emotions - the desperate need for the goodbye to be clean and clear and final, with no unanswered questions. But you are not going to miss him any the less, or hurt any the less. And other questions will come up - other thoughts and worries will need to be answered. If you choose to break contact with him, somewhere a line has to be drawn. No matter what happens.
If you cannot bear to let him go, then you need to finish your relationship with DP and be honest. But if you want to stick with DP, all of this has to stop. Seeing him again to say goodbye will be a dangerous indulgence.
I am saying all this because I hope that you can avoid getting yourself in any deeper to a situation that will hurt you.
You say that your DP is away this week and therefore missing the couples counselling. Does he realise how serious things are for you?
Perhaps call the counsellor and ask her about the wisdom of going along alone? She is the best person to speak to about the ethics of it. I do think you need to tell DP about your having had feelings for someone else. It doesn't sound as if he is aware of how desperate you feel.