Why do people often find it very difficult to go the miles again when the last relationship keeps bringing nothing short of broken dreams and painful yester' memories?

Usually it's the bond & special moments they had with that person, you give your all but in the end it fails so a part of you tends to be broken too...emotiinal pain could sometimes feel worse then physical pain

Very true. But it has always curropted the minds of such people who in turn fail to see what better things awaits them

- Their last relationship wasn't that long ago and getting into a new one was too soon.

- Their last relationship lasted long enough for pair bonding to occur the ending of which is losing a spouse, even though no magic government recognized ceremony was performed, so the last relationship was far more traumatic than others.

- People handle trauma differently, have different thresholds of dealing with it.

- Some people don't want committed responsibility generating risky relationships. They get into bad ones guaranteed to fail in order to prove and justify why to stay out of them. People are still group creatures so crave sex, contact, companionship, group safety, and social relevance.
They get the benefits they want for a while, then sabotage it to (in a sense) not have to "pay" for it using past harm as a scapegoat.

- Previous relationships never provided some kind of "better things" or not the magical feelings some people seem to expect, so there's no experience of relationships providing anything all that positive, which creates less incentive to try, more focus on how relationship don't offer anything but broken dreams and memories. So their mindset isn't "push through for the treasure," but, "protect the treasure you have, wall up, and defend from attack and pain."
Their abilities and perspective aren't geared towards "going the miles."

- They've never "gone the miles" before, nor seen anyone that has, so have absolutely no idea how to do it or what it means. When people are faced with stress and adversity and don't understand it, they look for reasons to explain it. Blaming past relationships is more attractive to people than sitting around, self analyzing, and coming to the conclusion "the problem is me and my inabilities, weaknesses, and insecurities, and I have to change."
Easier to blame an immutable past that can be held as separate from themselves. It means they don't have to change, and it's not their fault, that way.

- They're looking for a way to get out of the relationship that avoids a lot of conflict and judgments.
"It's not you, it's me."
"It's not that you are boring, have a small penis, smell bad, and are poor, it's because of my past, I've been hurt, so it's not you, it's me."
They don't find it difficult to "go through the miles," they just don't want to, with you.

Pick one that makes you feel the best.

MOST PUT THE BLAME ''OUT THERE'' AND NEVER SEE THERE PART... ITS COMMON NOW TO JUST GO TO ANOTHER BOX AND AND START OVER. THERE IS A LOT OF PAIN CHANGING BOX'S....I WAS SO HAPPY TO SEE MY PARENTS FINALLY BECOME CLOSE FRIENDS BEFORE THY PASSED....

Some people carry their 'baggage' in the next relationship and miss out great opportunities