Practice, practice, practice. I know the frustration. I thought I was ready to start on my first frame after doing about 10 90 deg joints on .035 st scraps. Decided it would be a good idea to do some test ht and bb joints with the real tubing sizes and geom that is planned for #1. It payed off, because I badly burnt the first st (.025), second was much better, and third time was a charm. My heat control and filling still gets better every inch I weld.
I got really worried about speed to my detriment. I finally started slowing down and really paying attention to the puddle size and hitting the center of the puddle with the filler. Things started to click then, and the welds still had decent color, no grey.
What size filler are you using? I've been using both .045 and .035. For a rookie, I do better with the thin stuff using .035.