How often are you feeding it and with what? 15 gallons seems a bit big of a bag for that size tree, Is the soil always wet? Pull the tree out and check the roots for rot.

Why the bag?

Otherwise if the bag isn't filled with the same soil as the ground around the bag consists of, is instead some kind of potting soil like mixture then that is likely to be the basis for the poor condition of the tree that has developed.

it is clearly underfed and/or poorly drained... Spring is coming. Treat it well and it should respond

I planted the tree in the bag because we will be moving in six months. I figured it might be difficult to dig the tree up from the ground. The soil in our area is mainly sand, so I used potting soil in the bag. I do feed the tree with citrus fertilizer once in a while, and water almost every day if it hasn't rained.

I see some speckling white dots indicative of spider mites past or present

I wonder if it's citrus greening.

Commenter mentioning citrus blight might be onto something.

Definitely watering it too much, even if that's not the primary cause of the problem. As trees and plants roots actually need dry time. Since when wet, it's difficult for them to intake atmospheric oxygen, and tips the balance toward soil organisms that favor oxygenless conditions. Soil in pots dries out quicker, and a potting soil is preferred over dirt dug up from the ground.

Fertilization actually attracts browsers to lusher vegetatively growing leaves. Application of fertilizer should end at least a month before a tree goes into dormancy to prevent frost damage on newest grown tips that had no time to harden off. May not be a problem where you live down south compared to northern latitudes.

Best diagnosis tool is observing any change to your leaves. Especially Browning around outer border, overall color, vein color, curling or cupping, spots or bumps. If it's affecting only the new leaves, or only the old leaves. Have you moved it around and several different locations or bring it in the house at night and back out in the day?

Temporarily healing it in the ground would prevent it from drying out so quick. And a tree that's temporarily grown in a pot or a fabric bag will require some root pruning before permanently installed. In fact most commonly accepted practice is to wash away all the soil and plant as bare-root.

Since root direction could be suspect, especially if was planted in larger pot once or twice before. As all Roots need to be pointing out bound like spokes on a wheel. Prune any circling or kinked roots back to where they were still growing straight away from Taproot. Think of how guy wires extending out with all the slack pulled out of them hold up a radio station antenna.

By planting bare-root, you can stretch each root manually outward horizontally. Look for top most set of roots and plant them no deeper than 1/2 inch below grade. Above that point is where creamy brown tissue transforms into green above-ground stem tissue. This is where your trunk flare will develop. Most tree species only produce one main vertical tap root their first two years, then stops extending any deeper when lateral root growth takes over.

No need to loosen the soil along the bottom of the hole, in fact firm it down so tree won't settle. No one suggests soil amendments anymore, since we want roots to get used to the common soil they will always be growing in. Last suggestion is to water along the root balls perimeter to coax root growth outward where roots are not yet growing. Stake as low as possible just enough to support tree which allows trunk to exercise in the wind.

Actually, the tree does not look all that bad, we all have seen trees much worse. Basically it just looks to be under fed.

I would check the moisture in that pot, sometimes water can run out & despite regular watering, although the soil surface may appear moist, it may be dry down deeper underneath. I looks badly wilted and that would be the first thing I'd look at. Here in FL, potted plants often need watering daily

Though it's hard to tell from the photo, it looks also like it might be planted too deep...?