Are you getting a hump in the middle along the length of the board? This can come from applying extra pressure to start the cut and from applying extra pressure as you come off the board without realizing it. It is a useful exercise to let go of the knob completely before the nose reaches the end of the board and just push with the tote to convince yourself that you don’t need to push down to continue the shaving. Then, learn to deliberately unweight the front of the plane as you come to the end of the board. A different grip on the knob can help, too. See, for example, https://commonwoodworking.com/wp-content/uploads/2018/02/P1000047-1.jpg . Or, rotate the front hand so that the pinky is on top of the knob and the thumb and forefinger are pinching behind the knob (probably on the cheeks). Your elbow will likely be up in the air. This is an “old fashioned” grip you see in old photos sometimes. I find it reduces the tendency to push down at the end of the stroke.
Extra pressure entering the board could be a habit, but could also be a sharpening issue.
All this being said, even when your skills improve, there’s still a tendency to plane a hump into the length of a board. So, it pays to learn how to form a hollow. Simply put the plane down such that the mouth is about 1/4 of the way back from the middle of the board and push forward until the mouth is about 1/4 of the way ahead of the middle of the board and lift the heel of the plane as you push forward. This lift will cause the iron to come out of the wood and nip off the shaving. Repeat. After a few strokes like this, the plane will stop cutting because you’ve cut a hollow into the center of the board. It only runs for a short section of the board, though. You can extend the hollow by extending the cut a but back and a bit further forward progressively.
If you play with this and get a feeling for how it works, you can start using this to counteract the tendency to plane a hump into the length of the board. Periodically make hollows in the middle, then return to working the length. You will find, when you go back to working the length, that you get a shaving at the start of the cut, then no shaving where you have the hollow, then a shaving on the other side of the hollow. As you work down, you will get a full length shaving. At that point, you will likely start making a hump again. If you have more thickness to take off, make another hollow and repeat. Don’t read this as a recipe that says you make hollows and then take off the ends repeatedly. I’m just trying to explain another pattern of planing you can use to monitor the state of the board and to adjust how the work is progressing.
If I want dead flat, one of the last things I do is to reduce the plane set to a fine, fine shaving, put a hollow in the middle, extend it, and then work end to end until I get a continuous, fine shaving. I then stop. I can’t get flatter than that.
As an aside, the more I plane, the more I find that I work on the bench top against a stop rather than in the vise. It is faster for me and the board is fully supported. It eats up bench space, but my bench is long enough and I now consider this a requirement.