Hello Hawkz,
I had a similar diagnosis to you March last year, T3b SVI, Gleason score was lower at 3+4, I had some very dark days to begin with and its horrible. I'm not sure what treatment you will be offered, I would think most likely Radiotherapy and Hormone therapy. This is the route I chose, though I was given the alternative option of surgery as I was relatively young at 48.
Like Adrian says you do cope and its amazing what you can and will do. A lot of the anxiety is not knowing, but there are lots of stories and help here so ask about anything that's worrying you or you're wondering about and someone will more than likely be able to answer.
The plan for me was four weeks of External Bream Radiotherapy (EBRT), with weekend off and two years of hormone therapy (HT) that started 4 months before the EBRT. It was changed slightly in the end to 3 weeks EBRT and HDR brachytherapy.
I found it all very manageable, I worked throughout with my EBRT sessions pretty much all in the evening. There are side effects from the HT, though luckily mine have been mild. Same with the radiotherapy. I have one year of HT to go.
I think its best to take everything one step at a time, I felt exactly as you do now, but the dark thoughts do lift, your life will go on. I felt so much better once a plan was in place for treatment.
I do wish you all the best, I know how horrible it all feels, as does everyone on here
John