1400 rating in 4 months, how do I get higher up?

Hello!

I have been registered on this site for about a year, but I just recently started playing chess on a regular basis. I had about 10 played games 4 months ago (blitz), but sometime in May I decided to start playing chess and became really interested in it. When I first started off, I have absolutely NO knowledge of chess and quickly my rating dropped to around 950. I stayed there for a few days until I got up to 1000, and a few weeks later I was on 1100. Sometime in July I reached a 1200 rating, in August a 1300 and now I'm on a 1400 rating.

I have done all of this with no studying so far (except for learning the only opening I know, the d4 opening for white). Also I have played quite a lot (about 2000 - 3 minute blitz games and a few 5-10 min games). I have about just as many wins as losses and it feels like it will be harder to improve rating-wise from now. I am stuck at around 1350-1400 (even though I often beat people around 1500, and even won a 1900 guy in an unrated game) but it feels like it'll be hard to improve.

I have tried some tactical solving but I feel like that isn't what I need to improve, I'd say my tactical skills are pretty good as well as seeing traps etc. I might be wrong though, so I'm curious, what should I study, and how/where? It feels like I'm at a point where I cannot get much further unless I pick a book or something similar up. I have tried a book but they're so boring with these endless made-up positions and all these notations...it feels like I'm not gonna get anything out of it. And what openings should I use for black? I pretty much improvise the black-opening everytime I play. Sorry this is my first threat here hope you guys understand, thanks in advance!