Can ex-lovers really be friends?

Answer 1

It's really challenging and not worth the effort because the injured party will always keep hope of rekindling things and WILL get possessive/jealous irrationally whenever you will be with someone else. It's not good for you, for your new love interest and for your ex.
Also beware that the ex might try to sabotage things, with small acts that can really be destructive in the long run.

A clean break is the best solution out there.