The main thing I'm missing at the moment is learning by osmosis from people with more experience. Learning stuff that I didn't know I didn't know. Mentorship.

Something I've been doing for general feedback is keeping my friends updated on what I'm up to and asking for their perspectives. There's a bit of a balance though cause you don't want them to associate you too strongly with your work and bring it up whenever you see them.

If you mean technical feedback then yeah not being surrounded by other engineers you can bounce ideas off kind of sucks...

Not sure how sporty you are but I have a pretty fixed weekly routine where I do sports with my friends some nights after "work" and I've found that great for forgetting about my project and pulling me away from the computer at a reasonable hour. For loneliness during the day I've found working somewhere busy helps, like a library. Maybe a nice way of framing it is that you can't get distracted by coworkers if you don't have any :)