There is comfort in what we know. And unfortunately if we have known abuse for a long time, we end up gravitating to people that display the patterns we know for above someone who would be more reliable. We know what every action/expression means and what to expect from each. It is not a conscious thing so it takes a while for us to even realise that is something we are doing.
Also when you are lonely, you are not in the best state of mind and any bit of attention, any compliment means the world to you and you want to do anything to keep that person and show them that you are worth their trust.
Sometimes it takes experiencing the same thing over and over before you can get off the hamster wheel. Sometimes it takes just one person giving you an overwhelmingly bad experience that shatters what you held until then as immutable truths. The end result is that once you start figuring the common patterns, it is easier to see them pop up in future interactions. You get the flags. You also learn your weak spots.
If you are guilt-prone, people will probably try to guilt trip you to get their way, so they will use similar wording and excuses to get you to agree with something you are uncomfortable or do a lot more for them that what they would be willing to do if the roles were reversed.
At one point I need to hear this, so I'll say it here in case anyone needs to hear it today:
It is okay to disappoint people.
You don't owe anyone your trust.
People who are worthy of your trust will not demand it from you, they will naturally earn it in time.