If your workplace has an affiliated or recognised union, ask if they can send someone in with you to support you. Even if you're not a member, they may be able to help - I do this for my union, and have been into HR meetings with non-members.
If not, see if you can find someone to take in with you - preferably not your DP, but a friend who you know is calm under pressure. It can be quite difficult to do this alone.
Write everything down. A timeline of incidents, key dates, things you want to refer to. You said you'd previously spoken to HR about your manager - do you have a record of what was said previously? Is there an anti-bullying policy, and can you get hold of a copy?
Write down what happened to make you walk out - you want to have your story straight and clear, and since it can be difficult to express yourself when you're (understandably) upset, writing it down in advance will help you get your point across.
Take a bottle of water and some tissues. Call ACAS this morning, they may be able to offer some useful advice.
The aim is to come across as a professional. You were under considerable stress from bullying and so had a bad turn, resulting in severe anxiety and depression. Your GP has signed you off as a short-term interim measure until a better long-term plan can be put in place. It's professional of you to be seeking a constructive long term solution. What outcome do you want - your job back, a different line manager, mediation, a severance agreement and good reference? Do you want to raise a formal grievance against your manager? Is it likely to be successful? Could your office feel that there is some substance to her complaints about you, and can you counter/address them? I don't know what your senior boss or HR are like - do you know if they're likely to be sympathetic, or stone-faced?