I used a red candle, dried rose petals, marjoram, red ink, a piece of paper that my ex wrote on, my blood, and my rose quartz pendant. I wrote his and i's name on the paper, held it with the crystal and then burned the paper with the candle and released the ashes to the wind. I've got a sudden gut feeling that I need to undo it, how do I do so?
My advice is that what is done is done. Let it run its course, and perhaps address symptoms of a mistake in the future, should any arise.
I wonder whether your gut feeling is intuition or some deeper feeling about your relationship with your ex, knowing it's a bad idea. Perhaps cutting cords* can be an option, though it is contrary to the spell done. I don't think what is done can be undone, but I do think some precautions can be taken. In this case, cutting the attachment can help avoid continuing contact with someone unwanted in your life.
*One of my favorite cord cutting spells as of late involves two taper candles with a string tied between, to represent the two people involved and their connections. When the candles burn down to the cord, it is burned and melted. The connection is severed. I suggest natural (jute, sisal, and cotton are the most common natural materials) as they will burn and not melt. And as always, do not leave it unattended, and always use a fire-safe surface.
If you knit or crochet, you could grab a length of red wool and start making a small patch. Doesn't matter the pattern you use, but knit/crochet three rows once per day for three days, making nine rows in total.
Run through the previous magical working in your thoughts, step by step from beginning to end as you stitch each row, ending the row when you complete the re-creation of the spell. Then begin the second row and do it the same way. And again for the third. Take your time with the process, reviewing what you did and examining the goals you had in mind at the time. Treat the process as a time of reflection and growth.
The actual length you use for your rows doesn't matter much, just as long as the row takes long enough to complete that you can mentally re-live the spell. Doing all this is a representation of building the spell into the patch, collecting and threading your previous working into the fabric. Think of it as sucking up the spell you worked.
When you have completed the ninth row in this fashion, three rows per day over three days, remove the needles/hook and bring the new patch of cloth, now having collected and bound up your spell, either to a plate of salt at your altar or outside to a natural place. Lay the cloth on the salt (or on the ground) and begin pulling the loose end.
Slowly pull and undo the stitching while making an affirmation of cancellation and release, repeating the affirmation as you undo each row, until you are left with the beginning knot on the wool that has been returned to a simple thread. Cut the original/first knot with your athame. (Or a knife or scissors, whatever you have.)
An example affirmation would be something like;
"What was made, be unmade. What was done, be undone. I release this binding with love. Return to your path in peace."
...If you don't knit or crochet, you could just tie simple knots instead, making a row of knots an inch or so apart, then doubling back and putting the new knot around the thread in the gaps on the previous 'rows'.