One thing I would like to add - maybe it was there just I missed it - is to tell yourself it is OKAY to feel sad. Let your feelings from from true thoughts. If it is true you loved or still love him / her - and a long relationship with a breakup initiated by the other makes it pretty likely - it is perfectly normal to feel sad. It is perfectly normal to not want to get over it, because you cherish the feeling of love even though it hurts now to a letting go.

Another very true thought here where feelings should flow from - I took it from my former Buddhist practices - is impermanence. ALMOST EVERY relationship ends badly: break-up or one of them dies. Humans are fragile and have a shelf-life of like 80 years.

There are these rare cases where they both die at the same time or like grandparents case where by the time grandpa died grandma was so demented that she hardly noticed. Even this are not really happy endings, a double tragedy cannot really be defined as a happy one, and seeing your loved one become an, um, "old fart" and yourself alongside has its own bittersweet sadness as well, I figure. Although we joke that 50 years later we will do wheelchair racing in the assisted home center but in reality we regret every year our relationship loses a bit more off that youthful sexual energy.

The lesson here is to start and conduct relationships with a consciousness of impermanence so there are no nasty surprises: it will almost certainly end badly. One will grieve over the breakup or death of the other.

OTOH I suspect that being conscious of impermanence plays a role in why I am in something like a constant state of light depression. It is sort of hard to get really enthusiastic over things when you know you will lose everything you cherish one way or another with very high probability.