Hmmm...
I can't write very legibly.
I dunno why but my handwriting has always sucked.
If I have to write more than a sentence at a time it will eventually devolve into chicken scratch that even I can barely make out.
I've heard this from a lot of people (in this thread but also friends): this is fairly easy to fix, and worth doing. There are some guides online (including youtube if you prefer that) on how to do so. Just take some time every day to write some stuff, imitating good handwriting. Just 10 minutes a day can do wonders - after a week or two it'll feel more natural and your handwriting will have improved dramatically. I was surprised how quickly it went for me. You have to somewhat keep it up after that, but even something as simple as grocery lists can do that for you.
Like most people here (I assume), almost all professional writing is done on a keyboard, but you'll feel so much better being able to write well. Everything from a guestbook at a wedding or some unexpected thing at work will make a little bit of effort pay off big time. Being able to look professional at work was actually the reason I did it, as I had to do an unexpected presentation which included writing some stuff on a whiteboard, and at the time my handwriting was like that of a blind three year old without hands.
Very happy I did it.
Also, if you haven't done so already, make sure your signature looks good. Presentation counts profesionally, even for something as small as that.