I doubt that that sort of spell to let go of anger and of the person you want out of your life will backfire on you. In fact it would be a very healthy sort of thing to do when a relationship ends badly.
Nor would I worry too much about undoing any love spell that you might have cast. Spells don't last forever, and if your relationship has fallen apart then it is likely that the earlier spell is no longer in effect.
That being said, there are some things you can do to help you end that relationship and move on. First of all, get rid of anything of your ex's that you might still have. Physical items can create psychic bonds for us and you want to get rid of those. Throw them out, give them back to your ex, burn them, or just get them out of your house by any means necessary.. Then I would suggest doing some meditation in which you visualize the psychic bonds (and anger is a bond) that may still be between the two of you. Once you can see them clearly visualize cutting each one with a knife or scissors and acknowledging that they no longer bind you. Try to let your anger go with the severing of the ties. Following the meditation, take a cleansing bath or shower with salt to wash away any negativity remaining.
Hope this helps.