The biggest difference that I see between people who can't heal from a breakup and the people who can heal from a breakup, is their belief system. If you have the belief system after a breakup of ‘oh that person is a narcissist’ or even ‘I'm not good enough, why is this other person that they left me for better than me’, then you'll stay stuck in that grief for a very long time.
However, if you shift your belief system to ‘oh this person just wasn't who I thought that they were’ or ‘I saw some red flags in the beginning of the relationship that I chose to ignore, so now I have to take the medicine for ignoring the reality’ or even ‘this person just knew that the relationship had ran its course and we just need to let each other go with love’. From my observation with my clients the people who adapted the mindset of the latter, were able to move on to more healthier fulfilling relationships in the future.