I'm still not sure how Warren would work out - I'm skeptical she's going to magically attract all the Sanders' independents back towards Clinton... however, I do think she'll at least be able to placate enough that it takes all the momentum out of any sort of attempt at further disruption and violence, which I think has the greater impact of damaging Clinton's number than just their lack of support in general.
edit: I still think Perez is the better choice, but I've shifted my opinion that I no longer think Warren is a bad choice
I went on a rant about this a couple of days ago. (Well...not rant, just kind of explaining my thought process.)
The only Bernie supporters Hillary needs to make huge efforts to win over could be swayed, I think, with the right Veep selection....especially if she positions it correctly. If she picks someone like Warren and makes it clear that Warren will be responsible for financial industry regulation or whatever, then...ya, that would placate some people. I also think it's a shortcut to party unity.
Also, my goodness. It's freaking May people. I can find you polls from 2008 that had McCain up by 10 in Ohio as late as July. Or polls from the same timeframe that had McCain up by 8 in Florida!
It's going to be okay. But, if you're worried, reach out to your local Democratic party and get involved. We always need help. If you can spare 2 hours a week between now and election day it could make a huge difference.