A couple things on the HPSP scholarship from a current recipient. First, I'm sad to say that there is no 20k bonus for clinical psychology. Second, in clinical psychology the scholarship is much more competitive than in medicine. There are 5 three-year scholarships (Navy), ~10 two-year scholarships (5 Army, I assume 5 AF), and 5 one-year scholarships (Army). About 45 people apply for each round of scholarships. I'm sure ROTC or other service will make you competitive, but I would have a back up plan in case HPSP doesn't work out.
You should definitely consider USUHS, but I would also go through the standard application process for funded APA accredited programs if it makes sense given your present commitment. If you do get the HPSP scholarship in a funded program what essentially happens is that uncle Sam doesn't pay for tuition since it is covered by your fellowship and you collect a stipend from your school and the military. If you go to an unfunded program you are on the hook for tuition until you (maybe) get the HPSP scholarship in the 2nd-4th year, and of course standard cautions about unfunded/poor match rate schools apply. Don't even consider unaccredited programs, which disqualify you from military/VA work.
For now, get as involved as possible in research with faculty mentors who can eventually write LORs, get some patient contact in if possible, and start thinking about your GREs over the next year. Feel free to PM.