First: In times of frustration about your child, always remember the parental mantra:
It's just a phase. It will pass. (And be replaced by the next "phase"...)
Don't despair, don't become impatient. You still have more than 80% of the time necessary for raising this child ahead of you. In a few years you will look back at the current phase and smile in the knowledge of the much bigger issues that are now still ahead of you.
I always find that a very helpful thing to remember.
From what you say, the most important thing to me seems to be
she just seems to be scared of the potty now
This, as you thought for yourself, hinders progress.
I'm not sure what I did wrong, or what I can be doing to fix this problem
I don't think you did anything wrong. The two of you tried, and it worked well initially. Then the child realized that this is hard work, and that it is now required to do this all the time, and so she withdrew. That is a very common pattern that I have seen a lot – with my own children and with friends' children. (A friend's son, after an incredibly well-going first week of kindergarten, plainly stated that this was all fun and nice for a week, but could they please go back to routine now.)
However, this is all empirics and statistics. It could also have been that what you did would have work out just fine and your child would be through with potty-training after two or three days. You never know. You will have to try and find out. And you will experience setbacks.
Now, the most important thing in my opinion would be to take away whatever pressure the child (or you – "I even took a day off!") might feel, and turn the potty back into something exciting that she wants to do. Remember: All children desperately want to grow up. They want to do things the way adults do. Material incentives (like your M&Ms) often work embarrassingly well with children, but, actually, underneath all this, is a much stronger and wholly immaterial incentive to grow up and be as cool as your own parents. The easiest way to raise a child is to find ways to tap into this urge.
Has your daughter noticed you're not using diapers? Nor anyone else older than she is? Have you talked about this? How important a part this is in growing up? Have you explained to her that being able to get along without a diaper is an important step for being (even more) welcome at her grandparent's and friends' houses? How your mother told you how you had to learn this, too?
As a practical hint: Many parents I know have found that getting rid of diapers worked best during a summer vacation, when there was no pressure "to get it over with" and when the child ran around naked and actually experienced what happens in the plumbing down there. And for some, it took two such summers.
Also: One of my sons never used a potty. He just refused, pointing out that all the other members of the family were using the toilet. So he used the toilet seat addition we had for his older siblings, rather than a potty. Of course, initially this was much harder for him (and, thus, for us as well), but in the end it all worked out well and of course simplified things for us, too. (He was also the one who never ate baby pap. He either wanted what we ate, or his mother's breast. That, too, worked out surprisingly well.) On the other end of the spectrum, one of my children would occasionally wet the bed at the age of seven. And there's nothing I could do about it except for being patient and fiercely loving the child.
To put it into one paragraph: Don't despair. Be patient. Keep trying to find a way. Relieve the pressure, from your child and from you. Instead employ your child's urge to grow up. Be patient. Remember how you love her.
And did I mention you need to be patient?