Legally, voluntary resignation will not entitle you to separation pay. Only when the employer-employee relationship was terminated illegally or through the authorized and just causes then you can be entitled to such.
It was made to show that you resigned voluntarily, that is why you were not given separation pay. However, if you can prove that you were constructively dismissed, you are entitled to separation pay. Go to the nearest DOLE office in your area, they will be glad to help.