I feel I've gone from decent to pretty good just recently, so here's what I've changed recently:
First, and most importantly -- think before you act. This was my biggest issue. Just slowing down and not rushing things has made me a far better player in just a few days. I didn't feel like I was really rushing anything either, but consciously thinking about which cards I choose (and how they interact with my deck and relics), and also which cards I play, has made a huge difference.
Second, keep all your relics in mind. For example, if you have a Kunai or Shuriken that gives dexterity/strength for every 3 attacks played in a turn, try to play three (or six, or nine) attacks per turn whereever possible. On the Silent, having a few Blade Dance makes a huge difference with these.
Build around your relics and their interactions with the cards and they will go from being a small bonus to being run-defining.
Third, look at good players playing and learn from them. Baalorlord is not both perhaps the best player in the world (by the measure of most A20 wins in a row, at 20 wins in a row, all including act 4), but also a great teacher and all around nice guy. -- EDIT: I linked to the losing run, changed to the last winning and corrected the number.
YouTube: https://www.youtube.com/@Baalorlord
Twitch: https://www.twitch.tv/baalorlord
I really attribute most if not all of my recent success to imitating what he does -- the other two points are ones I learnt from him.
There are other good streamers of course; I also watched a fair bit of jorbs streaming when I was getting started with the game, which probably helped me a ton with the earlier ascensions, and don't want to undersell him -- he's also had the world record, probably multiple times.
YouTube: https://www.youtube.com/@Jorbs
Twitch: https://www.twitch.tv/jorbs
Last week I had about 15 losses in a row on A20, with no wins, ever.
Since (and including) my first A20 win last week, I've had 6 A20 wins in 13 runs! Three Watcher wins (two in a row!), two Ironclad, one Defect. The most recent Watcher win was also against the heart, which was also the first time I've even tried beating Act 4 above A1.
Full disclosure: the stats above exclude all Silent run attempts as I'm still awful with her. Haven't beaten A18 yet and I'm usually not even close either; frankly I'm not sure how I even got to A18 as I seriously doubt I'd be that much better at 16 or 17.
(This comment is a bit of a test -- I can see the threads on lemmy.world but not any comments, so we'll see what happens. My subscribe has also been pending for a while now.)
Watch at least 1 Jorbs "Overexplained" video. He points out the considerations and calculations you should think about and that sets you on a path to self-improvement now that you know what observations you should be making.