ok, let this be a lesson for you not to rush into things for one. not sure how long you waited, but this is why many Wiccans go a year and a day studying before commuting to their path.
breaking a blood oath is possible, but you will be facing some negative effects for going back on your word. was this with another person you're in contact with or was this with a deity? this could effect the outcome, telling your friend/ex you need to undo something is challenging, but try breaking up with a god who isn't done working with you.
look into unbinding spells, as well as cleansings [you will need to cast a few times to undo and cleanse everything]
As has been said, you could try some unbinding spells but have in mind that blood magic creates a very powerful links between the involved subject. Some consider that that link can never be fully erased, but only attenuated.
Personally, I would see this through and look for ways which you can use to minimize the negative effects that it has upon you.
And in the future, it is my suggestion to not be filled with such haste to do something like this unless you have looked at all the possible outcomes that it can produce, weigh the cons and pros, assess your ability to follow through with it until you have achieved what you hoped to gain.