The complex word order makes it seem confusing but the base sentence is 'I was driven to despair' Add in 'myself' for emphasis/comparison 'I myself was driven to despair' (even me!) Add in 'more than once' meaning 'on more than one occasion' I was, myself, more than once, driven to despair. And finally add in 'to the very abyss of' as emotive imagery. Sometimes I find it helps to break down the sentence in this way. I hope it's helpful for you too and makes the meaning clearer.

It means, "I was driven more than once to despair myself," which is the same as to say, "I was, myself, driven to despair more than once." I hope that makes it a bit clearer.