Confirmation that my PhD is hard enough

The best protection is always to be working on hard problems. Writing novels is hard. Reading novels isn't. Hard means worry: if you're not worrying that something you're making will come out badly, or that you won't be able to understand something you're studying, then it isn't hard enough.

— Paul Graham, What You'll Wish You'd Known

I worry a lot, so I guess I can perhaps take consolation that my phd is a worthwhile problem. Two of my four great concerns at the moment are that the project does not contribute to society and that I am not capable of solving it.