OP - you have gotten some pretty good advice on here. Several coaches have chimed in which is really awesome. Kudos for them helping you out!
Lots of the advice you have gotten (while awesome) is very technical and targeted including my first post above, when I was 16 all of this would have gone over my head. You have a coach but you are looking for ways to improve outside of that system and this can be challenging. Regardless, it is frustrating to see regression in performance from one season to the next - so kudos to you for sticking with it and trying to figure it out.
Without knowing you and seeing your run, or knowing what your training has been like over the past 2-3 years we are all taking a shot in the dark. In my first post - I was really guessing that you are lacking aerobic fitness/base (not enough miles), but I don't have the full picture.
There is nothing I'm aware of that can transform a runner from a low 20's 5K to a low 18's 5K in short order, other than being born with talent and the right body type. I've seen several pretty talented runners (mostly freshman) go from high 19's/low 20's to high 17's or faster in their first season (2-3 months time span) - but those were kids who with no prior training could basically lace up and run 6:30 pace or a little better for 5K who responded well to consistent targeted training. It sounds like you were not too far off that trajectory your freshman year.
Before you do anything major I suggest you do two things:
Thing One: Determine if there is some kind of underlying issue or problem that is impacting your running and identify it if you can. From what I have read about you, I cannot pinpoint exactly what is going on - but I bet "you" can figure it it out and then determine the best path forward. It is at least worth exploring because you were a low-18 guy as a freshmen and maybe there is something fairly easy to address to get you back on that path.
WHAT IS THE UNDERLYING ISSUE? Many high school runners who experience regression or stagnant improvement have something else going on.
1. OUT OF SHAPE - If you didn't run over the summer (be honest with yourself) - that explains a lot, you probably don't have an aerobic base. That can be addressed but it takes time and commitment on your end. My first post was related to this a bit.
2. HEALTH/DIET CONCERN - If you are not feeling well (something is different) but have otherwise been training pretty well - go to the doctor and get checked out. Could be something easy to deal with like an iron deficiency, mono, diet change, or a hydration issue. Lots of other factors too consider like getting enough sleep, property hydration, is your diet OK (eating wrong stuff/not eating enough). Most of this can be addressed once identified.
3. OTHER SPORTS - If you are doing multiple sports and xc/track is not your primary that explains a lot too. If you are not training on a more or less year round basis then consistent improvement as a distance runner is compromised. While running is great training for most sports - other sports are not "generally" great training for distance running.
5. LOSS OF INTEREST/OTHER INTERESTS - many gradually lose interest in running and just are not into it anymore - could be due to school, work, family situation, girlfriend/boyfriend, or whatever. This is a pretty normal thing - i certainly went through this when I had kids.
4. PHYSICAL CHANGES - This is pretty common too. You may be physically quite different than you were when you were a 14/15 year old freshman. Hypothetically, if you were 5'11 140 pounds as a freshman and now your are 6'2 180+ pounds as a junior - this may explain a lot also.
Second: Talk to your coach.
I suggest you be honest about your frustration with the regression in your racing performances. If you can identify one or more underlying issues outlined above - that should be a big part of your conversation. I bet your coach is frustrated too and wants to help you get back on the right path. Honesty and respect will go a long way. If you want to increase your mileage and tweak your workouts I bet your coach will respect that - but if there is an underlying issue or two impacting your training/racing those also need to be discussed and addressed.
OP - good luck, I hope you get it figured out! Next time don't wait so long!