I have completed a PhD in theoretical physics (general relativity) several years ago. I don't know if you call it successful, since I am not working in academia (and there are no industry jobs in theoretical general relativity, save for a couple of people at NASA).

First, phdcomics is basically a documentary, if you take an average PhD student.

Second, unless you are very good and very lucky, at some point you will burn out and think of quitting daily or even hourly, the same way clinically depressed people think of suicide. There is an even chance that it will get better at some point later.

On the bright side, the feeling of discovery when you figure out something no one has before, rare though it is, is something else. And having it published and recognized as such is nice, too.

As for advice, one which is a bit late for you: pick an advisor who has a proven track record of seeing her grad students through and into a postdoc position, and the other is probably still relevant: on the bad days, just keep grinding and don't despair. Oh, and one universal advice: network.