Have you ever thought about the word relapse? If you look at the word's meaning, it is a temporary setback, not a permanent one. As for what you did, I don't think there is anyone on this forum who has not been there or done that.
As an earlier post, you start day 1, and pick up where you left off.
I personally can understand your regret for having given, I just spend over a month or two with the urges to do the same thing because of the effects on intensive therapy, and wanted to just do it and get it over with and feel relief, but my urges came with dire consequences if I gave in to my urges.
Don't let what you did add to your self-hatred. Take a pause, rest and think about how you have not engaged in self harm for a year, That's a huge accomplishment, I may be wrong, but I suspect there are a majority here who might find it hard to go that long without giving in to the urges. I myself can not say I have gone that long, in fact I can say I have relapsed multiple times over the last few months, while in each case the dire consequences that exist around my self-harming prevented me from actually cutting, I have went thru the motions all the way to having whatever cutting item I had against my skin with pressure. I had not have the consequences framework in place I do, my arms would be a bloody mess.
So the very fact you went a year, is a HUGE accomplishment, that others strive to match, build on that year. Process what you did, and do another year, even if you do this every year. That's a huge accomplishment. One I know I can't even come close to matching.