Hi there StepOn,
Welcome to SI. This place has helped me so much and I hope it can help you too.
Your statement that your AP was the one who insisted on NC makes me think that possible lines of communication are still open to him and you are trusting him not to contact you. Obviously that's not a strategy that's going to work. So, it's on you to block, block, block every possible means of communication. Phone, email, text, social media and whatever this app is.
Regarding the specific app, if you have deleted the app, how is it that you know he has tried to contact you? I'm also wondering about the business you and AP had together. Is that still a thing or is it shut down?
Beyond stopping all communication, the next step is establishing mental no-contact. That means redirecting your thoughts when you find yourself wallowing, ruminating, longing for or having intrusive thoughts and feelings about the affair and AP. Here are a few I used...I imagined a big red STOP sign. Also, slamming a big heavy door in AP's face and then walking away from the door. And, what became the most effective, shoving AP out of an open airplane door and watching him get smaller and smaller until he disappeared into the clouds below, then walking away from the door and returning to my seat. These sound weird but they are how you can rewire your brain. Each time you CHOOSE to redirect your thoughts, you change your brain wiring a tiny little bit.
Something you are going to need to do regardless of the outcome of your marriage is figure out how and why you were able to betray your BS like this. You say it was a desperate act, but wouldn't divorce would have been just as desperate? Somehow you found the destruction of your integrity and BS's trust and his physical and mental health as an acceptable price for having the feelings you wanted in the moment. The thinking that green lighted that exchange is what you are going to need to drill down on.
Difficulty in the marriage can definitely cause the temptation to cheat to arise, as can many other things that happen in our lives. We get what we don't want, or don't get what we want, and we are tempted to grasp for something to make us feel momentarily better in the face of those circumstances and the uncomfortable feelings they bring. But the thought processes that allowed you to act on that temptation, to actually follow through with the betrayal, those lie entirely within you and if you don't figure out what they are and take concrete steps to change them, this kind of thinking is going to keep following you around. It's great that you're in IC. Keep going with that and if you have a counselor that is in any way letting you blame the marriage for your decision to cheat, find a different one.
Something that helped me early on was to get educated about the true nature of infidelity and how it impacts the people involved. The book "Not Just Friends" by Shirley Glass gave me language and context to talk and think about the affair. The book "How to Help Your Spouse Heal From Your Affair" by Linda MacDonald is a fantastic resource for understanding what behavior on your part is likely to help reconciliation, and what is likely to hinder it. The healing library here on SI has many short writings on various aspects of infidelity. Most are from a BS perspective but early on I found them to be very helpful for understanding what my BS was likely going through.
This post infidelity landscape has some difficult terrain to travel. Even with both BS and WS wishing to reconcile, it takes a long time. Think years. 2-5 is the conventional wisdom around here, but in our case it was more like 6-7. You have to really, really want it and there is no guarantee of the outcome, in fact letting go of the outcome and getting comfortable with uncertainty is the best thing you can do.
With that in mind, it would be a good idea to get yourself some tools together for dealing with difficult feelings in a way that is wholesome instead of destructive. A mindfulness practice with daily meditation as its foundation is just such a tool and it has helped me a lot. I use an app called Headspace but there are many, many resources available in the world. Pick one and give it a try. The benefits are cumulative and it's one of those things that you can only understand by actually doing it, so don't go into it with the idea that you know what it should be like. Just do it every day for a short time (5,10,15,20 minutes) and judge for yourself at the end of 3 months whether it is bringing you benefit.
Keep reading and post when you have questions or just need support.
Welcome to the path from a fellow EvolvingSoul.