I would also recommend taking an psychostimulant to focus, but with a mood stabiliser and/or antipsychotic. Can't you read articles/essays because you have association/flight-of-idea problems or depression?

If you know you won't become full manic (I guess you should be able to tell by now, having been an undergrad) and are depressed right now, I would suggest just to try to take as much psychostimulants as necessary. Start with enough coffee or black tea and maybe add focusing/ADHD meds. I wouldn't suggest antidepressants, unless you really waste too much time in bed or on the sofa (not reading) without them.

When doing a PhD (haven't done one myself but I work at a university (not staff, just my own business) and have many friends currently doing their PhD), try to follow a very strict routine (alternating reading, writing and maybe, if it's part of what you do, experiment design and execution), I would join or start a group to together give presentations on what you are doing and to enjoy a—healthy and brain-supporting-large enough—meal together and/or frequent a cafe or go to some lectures just for fun, something to get your mind off things (don't be afraid of people stealing your ideas or asking difficult questions or give critique, or make sure they aren't in the group, since many like to keep things close to the chest which I don't see as healthy: it's just a thesis, a trial assessing your skills not your brilliant theories, not a magnus opus that has to be perfect for some reason), much like a conference (you have to get more used to that anyway).

So strict routine, fun and a place to express your ideas and (tentative) findings.

Of course you also need the reference manager my company makes, but I'm sure you will.

What kind of research are you doing: what field are you in? Linguists can get a coupon. Just kidding.

But other things that might be helpful do depend on whether you are doing scientific experiments or something in the Humanities (not being linguistics, in which you might do science). The friends of mine that do PhDs do thinks from research into mysticism and mania to scientific experiments and computational stuff, so I might give you advice based on how they handle things. Since intelligence is required for a PhD obviously, they are all either BP, ASD or bordering it (I am very serious about the causation. ).

Most get at least depressed, so really just talk about it with other PhD students.