I've only been scrolling about 6 months but I've learned a few things. First, this site is a treasure trove of good information. On here, I learned different blades from different manufacturers have different cutting characteristics. Some folks can cut a straight line with even a spiral blade. Not me. I mostly use a #5 or # 7 flat blades. I haven't even tried to cut intricate patterns that might require much smaller ones.
As to cutting straight lines, I cut really slow, use a head mounted 2.5 power magnifier and plenty of light. Lately, most of my material is 3/4 poplar. I find that thinner material requires more attention and a steadier feed rate.
Don't try to compare your scrolling to the CNC work you see in places like Hobby Lobby. That's an excersize in frustration.
Patience, practice and, more practice is what it takes. Especially if you're a novice like me. And, remember, sanding is an integral part of the process.