1. Not having self respect
Nothing ends a relationship, and allows the other person to walk all over you, like losing self respect. If you don't respect yourself then how do you expect others too? This happens all too often when one partner tries to please the other, at whatever expense.
2. Not listening to your intuition
If you're starting to see red flags, or something is off- you should not be ignoring it. Think about the fact that if something seems off, it is worth investigating even if it is just to clear your mind.
3. Short term pleasure/long term pain
Many people think about the time, energy, money etc that they have spent on a person and won't end a relationship because it hurts too much or too much has been invested. But what they fail to see, is that if things aren't working out now, you are only prolonging the pain in the long term.
4. Listening to words- not actions
Someone can tell you they love you, but if they are not backing it up with their actions and proving that they love you... then all you have is someone that talks the talk.
5. Being too trusting
Nothing wrong with putting your trust into a person, but sometimes trust has to be earner. This can be attained through various sources including time, and experiences. Trusting the first person that walks through the door is sure to lead to a potential disaster.