Patching drywall seams
Am I doing it wrong ?

Yes, there is no tape over the seam.
When two pieces of drywall meet it necessary to cover the seam with tape and mud, not just mud. If you do not tape it then the mud will crack eventually.  There is paper tape and fiberglass mesh tape.
The paper requires that  you to put down a layer of mud and then press the paper tape into it and flatten it out with your drywall knife.
The fiberglass has a stick back to it, you stick in place and then apply mud over it.
You then let that first coat dry and then you need to add mud and feather it out and away on both sides of the seam. It uses a lot more mud then you may think as you have to feather it out quite a ways to  make the raised seam blend in with the wall.
This is but-joint, there are many tutorials on YouTube on how to mud and tape joints and how to mud but-joints .
YouTube search for  &quot;How to mud and tape a drywall patch&quot;
YouTube search for &quot;How to mud and tape a but-joint&quot;