RockChicken - I know just how you feel, especially about the failed experiments!! I'm in my 3rd year, though luckily I have a funded 4th year too, cos I really need it!!
I just scraped through my 1st year assessment, failed my 2nd year assessment and was downgraded to an MPhil. I went through the whole range of emotions - grief, outrage, despair, giving up, defiance and grim determination to carry on. Luckily the latter won out in the end, I appealed successfully and am back on the PhD programme. Though that being said, the examiners said they still don't hold out much hope that I'll manage to finish, but I "might as well try"! 
My supervisors are utterly shite, they just pretend there are no problems and fail to take an ounce of responsibility for how badly everything has gone. All my experiments failed the first 2 years, so I'm very behind in terms of accumulated results. Plus I can't say I'm particularly interested in the subject I'm researching.
BUT! I will not let this opportunity pass me by! I've always wanted a PhD, and while it may not have a huge impact on my career progression (I've decided that bench science isn't for me and I'm planning to go into scientific publishing when I graduate), I could never forgive myself if I just gave up
It's incredibly hard, and the work and hours I have to put in are negatively impacting on me, my DH and my DDs. But I truly believe it will be worth it in the end. If I can do it, you can too!!