You should have a plan B. You cannot just go and say "I deserve a raise", because even if you do deserve it, your employer can still say "no". Given that scenario, what are you going to do? So, whenever it comes to "I need more money", my strategy is:

- search for another job

- be confident about 1 or 2 offers (maybe even go ahead and do a couple of interviews to be even more confident about your chances to switch jobs)

- ask for a raise as you would usually do (you don't need to mention you may switch jobs)