It's hard these days with social media unfortunately. You have your phone, with photos, instagram, facebook, texts, etc. It's really easy to just revisit all these things, sit there dwelling on them.

Time helps - one day you'll get through the day and forget that you didn't think of the other person anymore and when you do, you'll just be aware of that and quickly move on.

New person - if you're single now, then meeting someone new will always help. You get someone's number, a date, etc. and suddenly you're feeling great about it.

Activities - i started brazilian jiujitsu after a really crappy "breakup" with someone that ended poorly. Got in shape, had somewhere to go 4-5 days a week. It was new and exciting, but also difficult and required a lot of commitment. Plus it was just a social place to meet/hang out with people.