Hey, well done for seeking help to tackle your PND. It's okay to be scared. I haven't been through bereavement counselling myself but I know a bereavement counsellor. A lot of his clients have longer term issues, with previous bereavements or their relationship with the person they have lost which are brought to a head by a more recent event. He helps them deal with those and they can learn ways of coping. Maybe you could just have an initial meeting with a bereavement counsellor and see what they think - you don't need to commit yourself now and you will have a chance to see if you think it's for you. Do you agree with teh mental health nurse? Do you feel the past events are still affecting you?
I had bereavement counselling 20 years and four years after the events. It was when I felt ready, and it was immensely helpful, allowing me to go on and access further help and support. Several months on, this is the freest I've ever felt from my longstanding anxiety issues.