Don't sweat your non-invitation. When I went to Summer Seminar, I was in an element of ten students.
Out of those ten, only five were offered spots in officer training programs.
Out of those five, only three were offered appointments to an Academy.
Out of those three, only one went to USAFA.
That's right. We were a flight of ten students at the USAFASS, and only one of us made it into USAFA the following year... I'll bump that number up to two, come June, but that's because I reapplied.
This isn't to say that getting into USAFA is much harder than getting into Summer Seminar (in fact, stats might suggest it's easier?). It just means that being accepted to Summer Seminar is a pretty terrible indicator of your chances of getting into USAFA. Some elements had 8+ students end up at USAFA. Some elements only had one. USAFASS invitation and USAFA appointment are independent factors.