When painting with a roller sometimes it feels uneven, like the cross-section is an oval rather than a circle. What am I doing wrong

Don't stretch your paint.

The problem is probably not pressure (assuming that you remain consistent), but what I call "stretch". Folks tend to stretch the paint load of their roller or brush too far. With experience you'll learn what's the appropriate coverage area for a loaded roller.

The goal, of course, is to end up with a uniform film of paint on the wall. If you're working your roller out past its range, you're forcing the dry roller through the film and creating voids in it. Instead, try this:

1. Load the roller with paint. My technique is to pull some paint from the pool up the slope of the pan, and then work the roller into it a few times so that I have even coverage all the way around the nap.

2. Make contact with the wall 6-12 inches away from the newly-painted region. This drops the initial load of fresh paint on dry land, so to speak, and creates a temporary repository of paint.

3. Work back to the previously painted zone, overlapping somewhat. Roll direction isn't important at this point.

4. Work remaining excess paint from the repository outward, stopping when it becomes difficult to get coverage. This is the extent of coverage for that roller payload. Do not increase pressure to gain more coverage.

5. Give the newly painted area one more roll, all in the same direction, blending with the previous area. Keep an eye out for voids and runs, which should be rare. At this point the roller should be fairly dry and not leaving edge lines. If it is, roll the edges out on new wall to clean them up, then re-roll the working area.

Using this technique I get consistently good results. For most interior wall situations, a coverage zone for a loaded roller is about 2ft. by 3ft. Maybe less.

I also paint almost exclusively with quality 3/8" nap rollers. I don't mind a bit of stipple, which helps hide roller lines and wall imperfections.

Or you have other problems.

All this said, if you're seeing streaks after a third coat, something else may be going on. Maybe you're rolling too long after the film starts drying. Maybe your roller has residual primer or paint of another color in it. Maybe the paint needs better mixing.

That you have never seen brush lines before you have been very lucky. All of your ideas can affect how the paint goes on, cheap brushes, cheap paint are big ones but, the main reason I see for excessive lines is over working the paint so it won’t flow. when I do cabinets I don’t even try a brush but spray the paint and have used additives like floetrol a paint additive when I want glass smooth finishes. Sanding with a really light sandpaper will take the shine off, if I am putting a second coat on doors to get that perfect finish I will use 600 or finer paper wet to buff the door prior to the second coat if it is needed. I normally do this with clear varnish several times to get a real depth to the work, but not often on paints. You could sand the high ridges and reapply but make sure to over work the paint I see people put paint down spread it out then come back in a minute or 2 and run the brush over it again, this is when the brush marks stay or because of what I was taught is over working the paint.