I appreciated everyone's advice to fix my messy bellpull corners. This morning I flipped the bellpull inside out again and really examined my work.
First of all, I had left pretty generous seam allowances. Like near to 3/4", so I trimmed those back to a scant 1/2". The linen I used was quite stiff, and between that stiffness and the extra fabric in the seam allowance, my corners were doomed from the start.
As lots of you mentioned, I had trimmed the corners on the diagnonal, but I trimmed them even closer.
Then I turned it to right side out again, used my smaller knitting needle to push the corners a bit, and now they're still a little "dull", but worlds better than they were before.
I tried pulling the corners from the front with a thick needle, but either I'm a spaz or a chicken, because I just couldn't see any different results. I think that's a skill that I need to see in person.
NOW...I have a winter bellpull to finish and even though my house has turned into one giant dust bunny (trust me...that's not as cute as it sounds), I think I'm going to strike while the iron is hot and finish it.
And now I'm going to head off to the sewing machine, where I must repeat in my head:
Right sides together.....Right sides together.....Right sides together