Would going NC cement her and AP ? Maybe so but cement crumbles
She has been sleeping with AP for a year . Your safe and stable is her cushion not her reason for R.
Would you really be able to get over everything knowing she came back because you were stable ? What if you became unstable ? She gets to have another AP .
We all have bad days . Some can be worse than others . You are doing great and growing . Work on being a bit tougher .