Firstly don't confuse FORGIVING with FORGETTING. It is natural human nature when hurt, to have 'flashbacks' as I call them, and go off upset or angry. It doesn't mean you have not forgiven the person, just that even the most devout Christians hurt when little reminders come into their mind of who and when and how they were hurt or scorned. Forgiving I find, is MUCH easier than forgetting, and I know from experience. I forgave someone after just a couple of days, only to have something worse thrown my way, and again forgave, though had a HARD time dealing with it. Then the third time was NOT the charm and very recently another hurt for me, again, I forgave but because I am hurt, and being a female I associate EVERYTHING with that hurt, I have an impossible time forgetting. Women think differently than men, we women associate things with bad experiences, such as clothes the person wore, what they were driving, where they were, even trivial things like what we ate that morning of the 'disaster'. Because of that it takes a LONG time to get over things and forget.
Pray for your mom, and know that it may be that she is having constant reminders or flashbacks of the hurt she has experienced, and perhaps talk to a pastor or someone about speaking to her and guiding her in the direciton of professional help if needed. I pray that once she gets to the ROOT of the pain or pains that only THEN can she deal with her emotions, let it all flood in at once, deal with them or it, and then move on.
God bless you for caring for her, you are such a lovely person.