I retained an attorney and have realized it was a big mistake, am I able to get reimbursed the unused portion of my retainer?

You need to take a look at:

1. your agreement with your attorney, and see how much of your retainer is refundable, and under what circumstances, and
2. any bills or statements you have received from your attorney, detailing what he's done, and how much you've been charged.

Those two together should show you how much, if any, refund is due to you.

The agreement you signed with the attorney should indicate the extent to which any retainer you paid is refundable if not used. The specific language of the agreement will control over the more general framework of the law. Unless the retainer fee is a "true retainer" (i.e. an amount paid to reserve the attorney's services apart and aside from the hourly fee), you may terminate the attorney's services at any time and should be able to receive a refund of the portion of the retainer not earned or used to pay for costs such as filing fees.

It depends on the terms of the agreement. Read it carefully and either way demand your refund. If need be you can take him to arbitration and dispute any fees refunded or otherwise.