Why am I not improving?

I'd really focus on avoiding blunders and then learning strategic ideas step by step. Try to play some tactics, they'll help you train to think some moves ahead and you'll probably avoid some blunders because of that.

Take a look at this game you played: de.lichess.org/wZd9w1eT#62

It is very unfortunate to lose such a winning position to an avoidable mate in one. There were a lot of strategic ideas, from the second move on. If your opponent opens up the king's diagonal (2. (...) f6) so early in the opening, that can't be good for him and I really could not resist to play Qh5+ in certain lines. I guess those ideas will increase with the amount of played games and trained positions, so keep playing.

Btw.
@jonesmh #5 7. Qd4 should be winning for white.

Focus on improving your game and not your rating. Sounds counter intuitive but they go hand-in-hand. If you want, message me some games your confused about and I'll comment on them.

@ChessIteration You are right. In the variation, I used pattern recognition instead of calculation. This is a trap in the Queen's Gambit, but with colours reversed, white played e3, which causes the trap to fail.

1. e4 e5 2. Nf3 f6 ?? 3. Nxe5

1. e4 e5 2. Nf3 f6 ?! 3. Nxe5 is more playable than you think, especially in blitz. Black has 3...Qe7, but even 3...fxe5 4 Qh5+ Ke7 5 Qxe5+ Kf7 is not as simple a win as it seems. Look at the database: there are some black wins! So 3 Bc4 is not wrong, it is a logical developing move. This game was relatively well played, but of course 31 Qg3+ would have decided in white's favour.

@tpr I guess you're right if black knows what he's doing. But I'm almost certain that, especially in lower rating levels, it is way easier to play white here and the probability for black to blunder increases massively. For example, after 3. Nxe5, in the database, there are 1458 games where black played fxe5 (and only 432 games with Qe7) , resulting in a 72% win quote for white. I mean after Qh5+ if black is not careful and plays something like g6 (played 516 times) , Qxe5+ is already looking very pleasant for white and it could get even better if black plays something like Kf7.

Also, I think Bc4 surely is a very playable and good move , but I'd still prefer the variations mentioned above.

@ChessIteration Even 4 Qh5+ g6 5 Qxe5+ Kf7 6 Qxh8 Bg7 is not as simple as it seems. I faced the 4...Ke7 5 Qxe5+ Kf7 6 Bc4+ d5 7 Bxd5+ Kg6 against a strong player in a blitz tournament, and I won, but with some luck. These bad openings are sometimes better than their reputation. I know a FIDE master who plays 1 e4 e5 2 Nf3 Nf6 3 Nxe5 Nxe4?! 4 Qe2 Qe7. I once saw GM Andrej Sokolov as black defeating GM Eduardas Rozentalis with 1 e4 Nf6 2 e5 Nd5 3 d4 d6 4 Nf3 dxe5 5 Nxe5 Nd7 6 Nxf7 Kxf7 7 Qh5+ Ke6. In an earlier game Tal - Larsen, Bled 1965 Tal did not dare sacrifice 6 Nxf7 and played just 6 Bc4. So in the above game I do not blame white for missing 3 Nxe5 and opting for 3 Bc4.

@tpr I don't blame him for doing that, I think it is a good move too, I just believe that the Nxe5 Qh5+ story would have been easier for him if he want's to improve positional chess knowledge because that diagonal checking is an idea that occurs in a lot of openings, sometimes even as black in the Bishop's opening using Qa5+. For example 1. e4 e5 2. Bc4 Nf6 3. d3 c6 4. Bg5 d5 5. exd5 cxd5 6. Bb5+ Nc6 7. Nf3? Qa5+ which loses a minor piece as happened in this game : de.lichess.org/SghCRPsY
(don't blame me for not seeing 16. Ka1 Qxc3+ and complicating the mate there please :D )

I just think that it's important to learn to check those significant checking squares for the queen when you start to learn playing chess tactically. I mean, @tpr seems to be a very experienced player but I guess those kind of checks tend to be interesting for those who want to improve their rating and have just started with that, so the thread owner might want to take a look at them (and other tactics) . It's basically like @Masquerade already put it, it's about improving your chess and thereby increasing your rating, it does not work the other way around.

Pro tip: You won't improve your chess by passively watching videos.

Also, analyse your games.

I play golf passably. When I practice, I try to hit all my clubs a few times and spend time on and around the putting green. Meanwhile, I'm surrounded by players who pound the driver and can outhit me by 50 yards. Impressive ... but once we're on the course I can beat them from the fairway and on the green.
Same with chess. If your opponent plays decent opening moves and knows the traps, you'll lose. A lot.
As other people have said, change direction: general principles, middlegame and endgame study.
Once that clicks, you'll be winning more.