Thank you everyone for the encouragement and tips! I think part of the problem, once I think about it, is that this is a quilt I'm making to memorialize/cope with a loss. I wanted to hand quilt because it takes longer, as does the emotional processing (the top took only an afternoon, and I could have machine quilted it in another) but then when the stitches weren't what I wanted, I felt like I wasn't doing it justice; not the quilt itself but what it represents.
Also, as someone else pointed out, I came from machine quilting to hand quilting, and it is so much easier to get perfect stitches when the machine does it for you! :)
I do think I'll try to mark some masking tape to help me even out the stitches, and having seen some examples of "big stitches" also helps because it gives me a broader idea of what can be considered "good" quilting (but those were still much straighter).