At the moment you have got excitement (OM) and safety (your partner). You are going to have to pick one, and let go of the other.
Up to you of course, but your partner has stayed through thick and thin and worked on the relationship after the affair. So what he feels for you must be real. And as for the not noticing you enough, while it sounds a bit immature of you, if it bothers you, work on it.
As for OM, he likes the excitement, clearly. Loyalty is not his strong point. He has come back for more even though he's now married. Choosing him is a leap in the dark. Actually, it isn't really. He might not want a proper relationship with you and if he did, you might find he would simply get another mistress to create the triangle he obviously favours.
But really, this is not about 'Do I want Mr. A or Mr. B?', so stop thinking about the pair of them for a moment and ask the better question 'Why am I doing this?' and really think about the answer. If possible, do it away from home e.g. on a visit to your parents alone for a weekend, and without having contact with either man. Stop 'dabbling' with OM.
There is a lot in here to untangle: self-sabotage, being hooked on excitement, needing attention, denying to yourself what you are doing, as though you have no control and it's just happening to you, how you see your future. Maybe you shouldn't be with either of them, I don't know.
But please, don't be that woman with the hand-wringing, the dramatic 'Don't know what to do!' etc etc that ends up with protestations you are the victim despite treating everyone badly.