You wouldn't have thrown away anything but time with your current department. I didn't even become a fireman until I was 27 and didn't become a paramedic until 28. I wish I was your age when I had discovered an interest in medicine. Now I'm staring 30 and finally feel like there's light at the end of the tunnel. I'll be applying to schools next summer. Being a fireman is a great career. Pension, opportunities to promote, generally well liked in the community you serve, station camaraderie, opportunity to pursue other interests on days off, but it also comes with plenty of sleepless nights, time away from family, missed holidays/birthdays/games, and the job itself is inherently bad for your health.
I'd recommend sticking with it until your wife gets out of nursing school, taking what you can comfortably handle. If that's only 4-8 hours a semester, so be it. When she gets out and lands a good paying job with benefits, you can go to class full time and work part time if you so desire. I'd also put off having any kids, if you don't already have them. If for whatever reason, things don't pan out, you can always go back. Having a structural firefighting cert along with being a medic almost guarantees you a job in Texas.