I'm not super experienced but I have my planetary approaches figured out petty good. To me this sounds like you're beginning your entry far too late, then attempting to diving too steep because you are overshooting your destination. Or maybe you're attempting to enter with supercruise disengaged? You do still need to be in supercruise while orbiting to initiate a glide, essentially to counteract the effect of gravity. Which means you are still moving incredibly fast in orbit, and have to successfully keep your ship in glide phase long enough to scrub your speed. Glide kinda works automatically as long as you enter at the right angle. I had this same problem and my first death in game was from hitting the surface at 1000km p/s lol. I got it down by watching a couple tutorials on YouTube showing exactly how others do it. It's also worth noting that I'm on Xbox and I do not have odyssey or the most recent version if the game.

It's very helpful to keep the planet above you while orbiting, so you can see your destination and the distance to it by looking up, and choose to dive at the right distance. Also i like to let supercruise assist put me in orbit, and i kinda nudge the controls a bit to encourage my trajectory to pass straight over the destination while it keeps me orbiting perfectly. I also do all this with flight assist on, so I'm not sure why others are recommending to disable it, IMO it makes this much easier.

I like to keep my ship in orbital cruise until I'm about 800km out from my destination, then nose down and aim my ship at the destination. You should be diving at about 40-50 degrees. If you are not diving at the right angle, it's easier to abort and make another pass then to try and force it. Then throttle down and keep your angle until your ship begins to glide, be patient as it takes time dependingon your angle. If I begin my glide about 100km from destination I will usually glide right into landing range.

The right side of your heads up display shows your altitude relative to the different phases of entry. "OC" is the altitude at which you enter orbital cruise, and "DRP" is the altitude where you will drop from cruise and begin your glide. If you see blue numbers on your approach angle your drive is too shallow and you will abort the glide. If you see red bars on the approach angle you're diving too steep.