Resist, Retaliate, Press Forward!
I was in the same boat as you (no pun intended) many years ago.I tried this program twice when I was active. I thought I was pretty competitive but the people who usually get selected from this program (besides nuke) were much more competitive than I was. After 2 tries, I thought about reenlist and giving another shot but I decided to get out and finish my degree with the GI Bill and aim for OCS, which I still think probably the smartest decision I've ever made because OCS has more seats and frequencies while the STA-21 happens only once a year and is ridiculously competitive. I still recommend to try while you're active but if you don't make it and still want to be an officer, I highly recommend to follow my routine unless you're thinking about LDO or CWO.
Back in 2010, I had vied to apply for the STA-21 program as a prospective NFO Option Candidate. By the time I got most of the required documents on my Admin Department’s desk (around July, while aboard the G-Dub during my 2nd WESTPAC Cruise), I missed the deadline. I was devastated.
10 years fast forward, I am now on my way to OCS with a Pilot Option Contract. Guess everything happens for a reason.