First of all, as an English native I can tell you that Grammarly is dreck. =p Perhaps you would benefit from making a chess buddy who is 4-500pts above you as a coach, although if you do this, be very careful both not to impart or absorb bad habits, since a 1900 player has many faults too.
It is hard to quantify what 'biggest mistakes' means because we all evaluate our strengths and weaknesses differently. Some might focus on macro v micro [ie planning verbally like Capablanca, or Silman versus concretion calc and feedbacking like Botvinnik and modern players, or Soltis]. Some might look at the sport element of clock control, balance of skill in open v semi open v closed, technical elements such as specific endings, and some further might have specific tactical or positional blind spots.
In my experience one can broadly generalise the primary weakness of class players. I say that having at one point as an adult been barely rated 1100 and having worked up from that point.
Class A [1800-2000] players often have a habit of inconsistent tactical and calculation ability. I have often noticed their biggest sin is TIME, so they may make some slow moves in an opening. Such as needless ...h6's, or fancy play that leaves them in trouble in open games. This is usually exploited by fast play and a cross between tactical nous and open attacking play. Some positional motifs also tend to be ignored - I still see A players creating holes with their pawns by bad analogy to the Sveshnikov for example.
Class B [1600-1800] players will tend to fall down to traps in an opponent's opening, will tend to struggle with obtuse calculation sequences [where both sides eat pieces, not direct x for x exchanges], and are prone to hanging pawns or sometimes pieces to small forcing sequences. Positional knowledge is spotty, and unforced errors are commonplace, often because of cognitive dissonance, where a general rule and a position-specific idea conflict and cannot be adequately resolved.
Class C players' [1400-1600] games, and at 1398 I'll presume to include you here, tend to feature coarse material blunders and the games have a more up and down quality by the same token - one player might win a piece for a pawn, but will often fail to hold on to that advantage. Outside of known openings and structures, play can often be less cohesive, more fragmented - the player has often begun to learn axiomatic principles and tends to supplant these over concrete calculation more often due to lack of experience. Where calculation occurs it contains many errors based on fears of ghost threats or misevaluation of static features of a position.
Without looking at your specific games [would you like to post both a few of your best, and worst games with some thoughts?], I notice you play a great deal of rapid/blitz games, 15 0 or thereabouts. In these time controls one tends to lack the time required to
-Calculate beyond superficial level tactics
-Play technical positions proficiently
-Keep the thread of a game without wild swings caused by thought process shortcuts [such as general principle axioms, also known as 'hand waving']
It would be advisable for you to get experience at playing longer games. 45+45 or 60 0 would be suitable at first. Make sure to give your best effort on each move and after the game, categorise your errors. Did you miss a forcing move in a variation, did you reach a position but not know the process to convert advantage, etc. From that you can after 10 to 20 games work out what requires most attention for YOU personally.
You could read dozens of strategical books but you might still end up blundering pawns or pieces in the ensuing games. Your best bet is to start with basics, so this means elementary tactics, mates, combination themes, and techniques.
http://wtharvey.com has mate in 2/3/4, come back to the same puzzles every few days to reinforce the patterns.
If it ever comes back, chess.emrald.net is a fantastic site for tactics training, but please be aware that you should totally ignore the timed element. Set the puzzles at 1100 and keep solving them, 10 to 20 a day, take a few minutes per puzzle if you need. Scrapbook the ones you get wrong and note the themes.
chesstempo.com/chess-tactics.html has the same feature if you set tactics to easy and has both blitz and untimed sections.
Beyond this, you should practice against a friend or engine, how to win a won position. Take the engine here, and set it at a good level, and try to win with just the Kings, pawns, and one extra Knight. One extra Bishop. Then the same but remove one of your pawns. Keep doing this. Watch videos on youtube and elsewhere about endgames, because if you learn how to operate the pieces like that, you'll find them far easier in a middlegame.