Never begin a new relationship until you've fully moved on from the one before.
Also never begin a new relationship with someone who hasn't fully moved on from their one before.
By 'moved on,' I mean any necessary grieving done, not preoccupied with the ex, enjoying life, feeling good about yourself, and optimistic about the future.
The best way to move on is to decide to be single for a while; not dating, not hooking up, no fwb. Some short term counselling if that appeals to you.
'Get back on the horse' is sound advice, but its best to let the cuts and bruises heal before you do.