How do I know I'm actually progressing?

Progress at this is slow and lumpy, which means it is genuinely easy to feel stuck during a stretch where you are in fact getting better, so the first thing worth saying is that if you are putting in real reps and not doing anything strange, you are almost certainly improving even when it does not feel like it. The rest of this is about how to actually see that improvement instead of having to take it on faith.

The most reliable thing you can do is keep a simple record of what you attempt and whether you solved it, tagged by difficulty. Over a few weeks the ratio of solved to failed should climb and the level you can reliably clear should creep upward, and plotting your success rate by difficulty over time will tell you far more than your total solved count ever could. On top of that, do a rated contest every now and then, whether that is a LeetCode contest or Codeforces and AtCoder when the timing works, because a contest drops you onto unseen problems under real pressure, which is exactly the thing you are training, and the rating that comes out the other side is a number that volume cannot fake.

It also helps to pay attention to how often you reach for editorials. If you are opening them on most problems, you are working above your current level, and you should drop down until you are mostly solving on your own before climbing again from there. Early on, before you have the standard algorithms in hand up to roughly level 5, almost any honest practice moves you forward, so there is no need to agonize over picking the perfect problem. And it is worth expecting the very beginning to be the hardest part of the whole thing, because that first stretch, before your brain is wired for this kind of work, is where every problem feels like a wall. After that it starts compounding in your favor, since the more you solve the more confident you get, and the more confident you get the faster the next ones come.