I am by no means an expert with curses (Lucifer usually talks me out of doing them) I welcome more input from other more experienced members.
I am the offensive/aggressive type so I am not sure about undoing a curse I would send out. But I suppose you could do this with candle magic. In theory all things are connected to us as we are all in reality one, so maybe use a candle and empower it to absorb and reverse all curses you have cast.
I would cleanse your chakras and energy body with light,infernal energy pushing out all the negative energy as black smoke or mist.
If you know their names, write them down or make a sigil representing all the curses you have cast or a sigil to represent all the people you have or think you may have cursed.
If you make a sigil you will need to open it with the intention of it representing all the ones effected by your curses. If using names, push energy into the paper and see it glow then state that all the names on the paper are no longer cursed and are freed from their torment (The statement of intent will come from you so you know what is best to say) these are just quick examples.
Hold the candle with both hands and empower it to remove and negate all curses that have been cast by you purposely or not and free those effected from their torment. Place the candle on top of the names or sigil. Light the candle and state your intention again but add the phrase " as this candle burns" as the first thing you say when stating your intention. Each time the candle is lit repeat the intention. See black smoke being pulled from the names/sigil going into the candle flame being burned by it.
Trust and know it is working. You will know when it is done by intuition or spirit letting you know, then get rid of the candle. Do as many times as you feel is needed.
This is what I would do if I was concerned about this.
I used this method to heal my grandma from covid-19 and it worked but I combined energy work and asked spirits for help as well. I worked on her for a few days and kept pushing energy into the candle. She lives in Chile so it was long distance healing.
Lucifer or Belial can guide you as well. If the pact you ended with Belial was civil and no ill intent was made, I am sure he will still help you otherwise contact Lucifer for guidance. They can teach us how to do many things but won’t always do them for us.