I started a PhD 6 months ago and already feel lonely and anxious

First and foremost, I've never struggled with depression or anything and don't think that I will in the foreseeable future. I have always been mentally stable and have never had many real problems, though I've always been quite frustrated with my shyness.

Anyway, my life has changed quite a lot lately. I've started a PhD in physics and I work long hours completely alone. I spend almost all day alone in my lab and the few PhD students left in my department (which is quickly falling out of fashion) are all very close to finishing their theses, so they are eager to get out of it as fast as they can and don’t show much interest in talking to me.

Besides, many of my old friends have left the city (or the country). That means that, for the most part, my social circle is restricted to no more than three people, and I can't even hang out with them as often as I'd like. I thought that I might meet some girls over Tinder, but I am usually so tired and anxious that I just let conversations die.

I feel quite lonely at the moment, but that’s not what’s killing me. It’s the thought of 4+ more years of this work (which I feel proud of, btw) with the same isolation, fatigue and occasional anxiety. I feel specially stupid when I find a girl that likes me but quickly loses interest when I’m unresponsive.