I know how you feel. I've had two back operations for scoliosis (curvature of the spine), multiple blood clots in both legs, plus degenerative disks in my lower spine. MDing for me is a painful experience, but like you , I love this hobby to much to quit. A couple things that may help.
1. You need to have your lower rod adjusted so you stand up as straight up as possible to eliminate the need to bend at the waist which puts extra pressure on your lower back/spine. The further the coil is away from your body, the more top heavy the machine becomes. If you are a tall person, you may need to look into a "tall person" lower rod.
2. Try to not look at the control screen while detecting. This adds even more stress on your back. If you've been detecting for a while, you already have a good idea as to the tones for what you want to recover. Use the screen to confirm the tone.
3. Instead of looking directly at the coil while sweeping, look just ahead of it at the next area that you are going to detect. Just this little angle change of your head will help you stand straighter.
4. I use the Swingy Thingy harness and will not hunt without it. Once adjusted correctly, your machine will be well balanced and the coil just seems to "float" above the ground. Really helps ALOT.
5. If I'm going to be on a long hunt, I take a "travel" chair with me. It is very light and can be strapped to your back without interfering with mobility or ability to hunt. It is amazing now much better your back will feel if you take a seat every once and while to rest and take a load off. Or, just take a seat on the ground.
6. Whenever possible, dig your plugs with a shovel vs. a hand tool. This will cut down on the amount of time you spend on yours knees and the numbers of times you have to get up and down to the dig your targets.
7. Medication will help, i.e., Ibuprofen, Acetaminophen, etc... Also, using Heat/Cool Wraps (ThermaCare, Beyond BodiHead are a couple brands) while detecting helps.
8. Doing exercises to strengthen your stomach muscles and lower back muscles will help take some of the stress off of your spine.
9. Excess weight in your stomach area puts additional stress on your back. If you are overweight in that area, losing a couple pounds will do wonders for a hurting back. Believe me, I know.
10. You indicated that you are young and in pretty good shape, therefore, your back pain could be an indication of something physically wrong. You may want to consider going to the doctor to get it checked out. Even if he does not find anything, he could prescribe some meds (Anti Inflammatory, Muscle Relaxers, etc.) that could help or offer other advise.
If I can think of anything else, I'll update my post. Maybe my list will spur some other ideas as well from the rest of the folks.
Hope this helps.