If a potential tenant pays the security deposit early to hold the unit, but then backs out, I consider the paid deposit to be forfeited as the rent payment for that first month. 

Do we just forfeit our security deposit and abandon our lease?

Each asset manager will have a different form that should be available on their websiteNo need to wait to move forward with the other property as the absolute worst case is you forfeit your earnest money.

The full amount is required two weeks prior to check-in.If they cancel 30+ days before check-in, we take a small administrative fee and refund the remainder.If they cancel 14 - 30 days prior to check-in, they forfeit the 50% deposit.If they cancel 1-14 days prior to check-in, they forfeit 50% of the total amount.

The easiest thing is to accept a march 1 move in and forfeit the existing tenants deposit, resulting in no out of pocket to me, but I also know I need to attempt to re-market for rent as quickly as possible, but it seems like I did that already, but just making sure I dont need to keep it listed when I can have this applicant put down a deposit ASAP which she has agreed to.

Or you can have a clause that specifically states they forfeit the entire deposit if they fail to occupy by a certain date.

A realtor works in your best interest.If I am you I would back out of the contract and forfeit the earnest, losing a little up front will save you years of a headache & hire a realtor next time. 

If for some reason the applicant changes their mind or does not have the money to move in they forfeit the deposit.

In a standard lease with possession given in the future the security deposit may be forfeited if the tenant fails to take possession on the date of the lease.

@Tyler Speelman I let my tenants know that they are forfeiting their deposit if they move out early.