Dear HTR,

Well, since you've asked many of the most basic questions about relationships, it behooves me to give you some precise answers.  And since I don't have any precise answers, I'll hit you with some analogies.

Relationships Are Like Cars

On the other hand, you don't trade in your car just because it is out of gas.

The sage older brother of my college roommate used to say, "A relationship is like a car; when you think there's something wrong, something's wrong".  I took this to mean that there are many underlying mechanical doohickeys in a complex contraption like a car.  Most people do not understand all the cylinders and brake linings, and transmissions involved in the day-to-day operation of a motor vehicle, but they are cognizant of the squeaks, squawks, and awkward handling of driving.

And when you think some new noise indicates a problem, you're almost always right.

As to why it's so difficult to leave a relationship that has all those squeaky noises and handling difficulties? - it's like potential versus kinetic energy.  Yep, we have to go back to high school to figure out the important stuff of life. 

Kinetic Energy

If you place a book so that it is standing on its thin edge, why doesn't the book fall down so that its wide title cover is lying on the table, which is clearly a more stable, comfortable, and safer configuration for the book to exist?  The reason is that the book has to tip over first; it has to be provided enough energy to get over that hump of a known, more-or-less stable situation, over its edge into the very comfortable, extremely compatible facedown position.

Moving On Requires Pain

We all have to feel uncomfortable enough to move out of a bad relationship, through the edge of the unknown, into a possibly good relationship.  And that takes some pain.

– Lefty