well it's good that the letter says she misses him, that means he is not seeing her anymore... it matters not what she wants or what she feels or if she wants to hang onto hopes of being with him again... as long as he doesn't want her and is there with you that is all you need be concerned with.
don't have to tell you to stay away from her, knowing that the letter bothered you would only give her power.... she is her own problem and as long as h keeps being there with you she will eventually move on.
I would assume that thoughts of ow or the "bad time" never actually dissapear completely they just fade until they are but an annoyance.
stay strong and know that your h loves you and wants to be with you.
LL