No one will come with me for a private 2nd opinion - should I bother going alone?

I had a very similar experience to what you are going through but I went through it this time last year. I was originally diagnosed the week before my 39th birthday with a 22mm invasive ductal tumour er/pr+ and was told WLE, rads and 5 years of Tamoxifen, but once they went in it turned out to be 34mm and had extensive lymphovascular invasion. Because of the size and my age I was told chemo required and was given the option of a mastectomy. I went into a complete meltdown on hearing those words, like you have, it is a natural reaction to have. But I knew I needed to get as much information as I could before making my decision, so I went back to my surgeon and asked her lots of questions and a friend wrote everything down for me.
My situation is a bit different in that I actually went back to my home country New Zealand for my chemo and eventual mx, so I got yet another opinion from a top breast surgeon there as well before I made up my mind. In the end I wanted the reassurance an MX gave of only 2% recurrence over 13% without, so I had an mx with immediate reconstruction. It turns out that the pathology from the mx showed a “complete response” and there were no cancer cells anywhere in the breast tissue. The reassurance I have from this makes the whole rocky journey worthwhile. I know it’s hard to imagine it now, but eventually you will come to terms with what you are going through, because you have to.
As has already been mentioned, you don’t have to have your op on the 6th if you need some more time, I think it might help you start dealing with this if you make a decision now to take control of the situation by telling your surgeon you need more time, and you want a second opinion. You also must seek help for your anxiety and stress levels, please please please ask your BCN for a referral or go to your GP. There is help available to you and you must take it. None of us here will tell you anything other than this is a tough journey to be on, it can be really crap at times (especially at the beginning) but it IS survivable and you too will survive it because we are all here to help you at every stage of this. No question is stupid, no complaint is unreasonable here and you are never alone through this.
Take care, and do keep posting so we know how you are getting on.