I think it is definitely possible to break up and then have a strong and committed relationship when the couple gets back together. But it only works if both partners feel heartbroken when they’re apart and genuinely feel as though they belong together.
If one of them just feels it is ‘easier’ than being single or gets back together out of ‘habit’, the relationship seems destined to fail.
Sometimes it is an issue of losing trust or respect (depending on the circumstances of the break up). Or perhaps the second time around just doesn’t feel as good or as fun as the first.
Sounds to me like you should bravely walk away and accept it isn’t working. No point staying in a relationship that’s only going to make you unhappy.