People tend to focus too much on themselves and not enough on the person they are negotiating with. Your employer will likely care more about what is in your employer’s best interest than why you need or want more money. Also, one needs to be careful to be persuasive but not annoying. Thus, “if you don’t give me a raise, I am leaving” may result in the employer replacing the employee, even if not immediately. On the other hand, “I appreciate working here and would like to make it work long term. How can we get my pay to at least the level of what similar people in our geographic area typically make for this type of work?” and have the data to back it up, would likely give better results. Your worth to the employer is probably the cost of your replacement plus the value of you over that replacement. Keep that in mind and show your employer why that is higher than you are getting paid.