Hi Lesley, I don't know you, but from your worries, you sound as if you are someone who is very concerned about hurting other people's feelings and this is a very good trait, however, it also means that you probably have a tendancy of putting other people's feelings in front of yours even when they don't deserve it.
I don't know your history with Mrs OCD, nor do I know what kind of work you have been doing, but I agree with everyone else that she will not 'change her spots'. Maybe you should look at her present state of mind (positive and happy) as a true sign that it is time for you to move on especially now that you have been given the possibility to do so. And when you go, you will do so knowing that you have seen her 'other/good side' and can address her in the future based on this knowledge that she is capable of being kind/nice and you can forgive her for whatever she did in the past.
For heaven's sake, don't stay for her, it wouldn't be fair to you and it wouldn't be fair to your heart. If I understand correctly, you are a nurse, so you need to share your kindness and goodness with people that deserve it and need it and that will thank you for it.
Go and be happy.