Getting better with perspective. As a few more tips, try drawing a vehicle of some sort. Boats and tractor trailers are easy because they have a lot of long, flat, boxy sections. Try drawing an over head cab Mercedes Benz tractor trailer or a 1969 Dodge Charger from a sort of corner view to get the hand of it. Next, once you're able to get an acceptable vehicle, try drawing a room from that same perspective. It's harder to try with the vehicles first, so it gives you more of a feeling for the angles you need. Conversely, it also makes it slightly harder. But don't get discouraged! Just erase and redraw the lines! I think what you're afraid of doing is showing a rough product, and feel that erasing smudges or blurs some of it. Don't worry about that. Most professional artists do a rough sketch, darken the lines they need, trace it over again, and then ink and colour it. You don't even need to do that latter half though. All you need to do is worry about getting those dimensions and that perspective. Keep going man!