You have come up against a couple of issues in mathematics study and perhaps both of them are causing you trouble.
The first issue is that many people have sort of natural "boundary", though not a hard one. But before you hit the boundary everything seems easy and fairly obvious to you and after that it gets hard. Hard isn't impossible but you have to "think different" to continue your progress. It is a lot of work. I learned fairly early that I had to work to learn anything and managed to do it. One way was to solve many more problems/exercises in a course than were assigned. A couple of times I just went into sort of trance mode solving a lot of similar problems. This led to insight into the behavior of real functions (Analysis, if you like). It also gave insight into relationships that I ultimately exploited in my doctoral dissertation. My sister was much better/smarter than I was and hit her boundary much later. Sadly, she gave up in the quest rather than learn to fight through it.
And yes, the level of abstraction increases dramatically in every mathematical field. Expect that. Work from the concrete (exercises) to the abstract (insight).
The second issue you've come across is that insight into math isn't universal. You can have deep insight into some math fields and very little in others. Partly this is due to the first issue, above, since it is a lot of work to excel in several math fields. Most don't do that. I also had some pretty good insights into topology and might have been able to do a dissertation there, but almost none in abstract algebra, even though it is heavily axiomatic like topology. Rings still elude me almost entirely.
So, if you want to excel in math, work hard in a chosen specialty. Don't try to learn everything at the same level lest that level be low. Work on a lot of exercises. Get guidance on your solutions. Try to abstract out the deeper meanings of how and why things fit together. Keep notes to record your progress. Keep notes as to where you need to make progress.
Sadly, though, Analysis is a bit past its prime. But, pick a field that you like and are comfortable in. There is some room still for good people in most fields, even some that have cooled over the years.
But, to really reach the top in a narrow math field, you need to reach the point where you can imagine what might be true based on what is known already. I don't think I'd achieved that level yet at the point of completion of my doctorate. It came a bit later, I think, though I had to move to another field.
Note that I don't believe nor mean to imply that there are people who can't do math. Short of a brain injury all of us can. You just need to learn how to work effectively (and hard) at it. Good teaching helps, of course. And the same is true in CS and perhaps some other technical fields.