How to create boundaries
In order to create boundaries, you need to learn to identify your needs and your responsibilities. What do you need regarding relationships with other people? Trust, respect, honesty, communication... These are the things that are required in order for a relationship to move forward. What are your responsibilities? Your own happiness, your own actions, your own emotions, your own well-being. Knowing your needs and responsibilities helps you identify your deal-breakers.
Your deal-breakers are things that violate your boundaries. They are the things that are contrary to your needs and responsibilities. If someone lies to you, for example, that is contrary to the things you need from a relationship: trust and honesty. If someone lies to you, they are not being honest. If they are not being honest, you cannot trust them. This relationship will also begin to interfere with your responsibilities to yourself, as being in a relationship with someone you can't trust causes most people to be unhappy. If someone takes advantage of your kindness, this is an issue of respect. It's very hard to be in a relationship or situation where you feel that you are not being treated as if you have value. People often start to feel resentful or depressed in this situation, which again, interferes with your responsibility to yourself.
It's important to remember that it is not the other person's responsibility to make you happy by changing their behavior. It's your responsibility to make you happy. Sure, it would be great if the person were not a liar or unappreciative, etc. but apparently they are. It's now up to you to do something about your own feelings. That is how you practice self-love and self-care.
It is not the other person's responsibility to change for you and chances are, they're not going to. You have to love yourself enough to say, "This situation is not best for me. I love and respect myself enough to walk away because what I am getting out of this situation is not enough. It does not reflect my value." The plain truth is, people do the things they do for their own reasons. You will not be able to change a behavior or a decision that has nothing to do with you. You cannot make someone value you or your time or your contributions if they don't. All you can do is recognize that they don't and respond accordingly.