I also think you should not be too hard on yourself over choosing to give Sweep a fighting chance. The reaction he has suffered is extremely unfortunate but not what the vet thought would happen, hence the change in treatment plan.
I bet that most of us would have done the same as you - I know that I would have - and I am praying that he feels better after the drip tonight and they can get him restarted on a lower dose of the chemo.
Big hugs for you too and lets hope Sweep responds to the new dosage xx