1. Reset your prices. (Routes - Price Management - Reset to default prices)
2. Invest in slightly more marketing, but don't overdo it.
3. Stop flying your two DC-8s. The overhead costs of operating these two jurassic (but awesome) planes are way higher than your profits.
4. Re-train your Very Large aircraft pilots to Large if you need them there instead, or to the smallest type if you don't need them.
5. Stop flying your three F-27s as well. Same reason as point nr 3. And close your Salt Lake City base. The costs of operating a base with only 3 turboprops is killing your airline. Or you could expand in Salt Lake City with more planes, but it doesn't seem like you have the money for it right now.
6. Instead of performing expensive D-checks on 24 year old 737-200s, try to swap them out on the used market instead. Often a younger plane with fresher D-checks is more economical to operate than paying for 8 million USD checks on old planes. Or you could buy some of my 737-200adv that are 18 years old with 5,5 years left to D-check for only 5,900,000 USD. That's cheaper than paying for your D-checks. Then scrap them before the 3rd check around 24 years. Or just lease them for 5 years.
7. Whatever you do, do NOT get a 4th fleet type. If you want to swap out your 737-200s, remove one of the other fleet types first. When you get a 4th fleet type, the commonality costs more than double across all fleet types. (Stupid game mechanic that I'm lobbying to change. No luck yet).
Good luck! You've survived so far, you'll make it to the end if you follow these tips.
P.S. Actually I just checked and I have 4 x 737-200adv that are 8 y/o with fresh D-checks that you can have for about 14.5 million USD. Lease if you like. They're all max range. Send me a PM if you're interested. I have more of them coming soon.