[eschewingmycud]

> Forget the outside stuff. ... Feeling like a failure or success is almost a worthless concern as you’re clearly nearly done with a huge life goal. ... Marriages are fundamentally just a societal complication of a relationship ...

Ungenerous translation: That thing that's been really important to you for four years -- just forget about it. If you stop feeling bad about it that you'll stop feeling bad about it. Also marriage is objectively a silly institution. Sounds like you'd like to get married. Maybe that makes you silly. Getting a PhD though -- that's going to change your life forever! You'll be proud! If you're not proud because you don't like your sub-field, well ... maybe that also makes you silly!

To the OP: Sounds like stuff's really hard right now. I'm sorry for that. Grad school is damn hard some times and relationships are too. If find the above advice reassuring or helpful, great! If not, try to brush it off and read other more supportive comments. I hope things get better for you soon.