Can I get a refund of my Social security contribution if I am leaving the US?

Since I am not going back to the U.S. again, am I eligible to get the money back?

Not if you were supposed to pay it. But depending on the conditions of your employment you may have been exempt from paying it to begin with (at least for part of your employment period). See the IRS page on this:

• F-visas, J-visas, M-visas, Q-visas. Nonresident alien students, scholars, professors, teachers, trainees, researchers, and other aliens temporarily present in the United States in F-1,J-1,M-1, or Q-1 nonimmigrant status are exempt from Social Security / Medicare Taxes on wages paid to them for services performed within the United States as long as such services are allowed by the United States Citizenship and Immigration Services (USCIS) for these nonimmigrant statuses, and such services are performed to carry out the purposes for which they were admitted into the United States.

You should reach out to your employer and ask them to refund the overpaid taxes. If the employer refuses, you can ask the IRS to refund the amounts withheld in error using the IRS Form 843. See details here. You'll need to provide the employer's statement to the IRS, or an explanation for lacking one, so start with the employer.

Note that there's a statute of limitations on claims for refunds, which may have passed for some of the years in question.