looks like you haven't sanded the other side of the line but it's probably low too. I'd lightly block the other side of the line to see where it's at. Most likely you'll have to spot in some filler. To do this just try to fill only what you need and leave your edges of the filler feathered with the spreader. you can do that after you wipe in your filler. That way you can block only on the bulk of the filler and when it's near level you can extend your strokes outwards onto the primer to blend it in. The tape line works well but will only be as perfect as how you lay the tape an easier way to get it started in the right direction is to **** your block at an angle and let your block ride off the edge by letting the line define itself. Doing that on both sides of the line will guaranteed get you a perfect line. The only draw back is that since you're sanding straight you can leave a slight flat spot you have to deal with. This is where hand feel comes to play and block skills come into play. Watch this video where I block the left door(14:30) and it will show you how to do it.
You can do what I suggested and then use tape and it will turn out perfect in a quicker amount of time.