You wrote:

Somebody that has been part of your life for so long, shared such amazing moments with can't just be forgotten, erased from our existence.

Absolutely relationships can really really end. Memory takes work. You have to water those memories with longing for them to stay alive.

You can sometimes remember that you had an amazing experience -- but you can't feel the feeling you had for that person. It gets kind of sad anyway, and you'd rather move on to feeling the feelings in real time with a new person who shares the feelings with you.

The issue I think is this: When our hearts finally open and we feel deeply for someone, we like to hold on to that big passion and do, until someone else comes along that we can transfer the big passion to. This is why, for some people, after a breakup they want to have as many new and random sexual experiences as possible -- just to loosen the hold that the big passion has on them. You start to replace it with smaller passions and heartbreaks which are more manageable than the devastation of losing the big passion.

Among the many things, loosely termed shit lil says is this: If you've had at least one big, crazy, wrong-headed passion in your life that doesn't end in broken glass, then it's been a good run.