I want to encourage a little more rigor in the evaluation of passed relationships. It is mental and emotional laziness to focus solely on the sour, and to fail to adequately appreciate and assimilate the lessons we were gifted in the relationship. The Gods put us in situations which provide opportunities for growth and learning, and relationships are not to be evaluated in terms of their pleasures alone, but often in terms of the productivity gained by their challenges. That a relationship has proven unsustainable does not mean it was unproductive. It is likely the other person stretched you in new ways, and gave you eyes to see things in a way you never did before. If not, the fault may often lie in the mirror more than in the other.
If time with a person helped you to grow, if a genuine giving happened, gratitude is appropriate. You should honor one who has helped your growth, even if it was at times through difficult testing.
We go way too easy on ourselves in these regards, letting our duty slip, and our associates, steeped in the disposable culture, are accomplices cheering on our emotional laziness, as we are encouraged to "leave it all behind", that hallmark of American denial. Leave it all behind? That is no heathen value! Rather, we honor the past, and draw out its genuine fruits to weave them into the blossoming of the present!
I say this less to excoriate and more to bolster and edify, for when we are lazy, we do ourselves a disservice. Be fair in your evaluations and judgements. That is all Baldur requires, but it is required, and is a high bar to match. Do your bitching and whining and tantruming in the company of friends or in your own privacy, get it out of your system, then gird yourself up, pull up your suspenders, and take a hard look. Put aside the "Screw her/him" reflex for a moment and perform an inner inventory of what you have gained over the course of your relationship. What did that person add to your life? We know there was plenty of material for manuring as well, but that is not the focus of this self-searching. You're looking over a particular period of your life with the object of appreciating your wyrd, grasping the lessons and strengthenings that, intentionally or not, you received. Be honest, because you're cataloging your store of resources, and you will want to make sure you continue to nourish and invest in those resources, to make good on the gains you got for your troubles. To fail to do so is to insult self, ex-lover, the Gods, and the Norns. Not a very prudent move. (No, not with shades of some kind of supernatural punishment, but the allowing of the natural consequences of self impoverishment.)
Be fair, and give acknowledgement where due. Gripes have a way of accumulating and burying the good : excavate. You'll be surprised at what you uncover and had forgotten. Sure, it might be hard work. You might have to go through a whole range of emotions and reactions. Work it through. Let your soul process what has happened, and draw gain from hard lessons and free gifts.
If the relationship wasn't completely abusive (in which case it is appropriate to keep them outside your law), see if in time a friendship can be salvaged. Don't be swayed by the easy maxims of the disappointed and lazy who say, "It can't be done". Do your determined, persistent investigation before you come to that conclusion. In time, past lovers can make great friends. Get over the sting and the bite, and they may prove themselves tried and true. Romance may not ultimately have been the reason for drawing you together. A different kind of fruit, more mild perhaps but still sweet, may come of it. Why turn down potential fruit? A heathen dares the difficult, and looks the impossible in the eye and stares it down, with faith in Gods who know "it is only a matter of time, skill, and wits" until all giants are ground down on the mill and made to serve some useful purpose.
Live and love in that spirit.