You have destroyed anti-aliasing. In low res images anti-aliasing is essential for apparently non-jaggy images. If you have non-antialiased sharp border in a high resolution image the jaggines cannot be seen in low res displays. The border gets blurred without asking, the jagginess appears only when zoomed in.
In the original image there's at the edge of the shirt a gradient mix between the environment and the white shirt. You have changed a great part of that mix to pink. There should be a mix between pink and the original environment.
If you do not have a high resolution image for the job try properly anti-aliased selection to make the pink area. The anti-aliasing occurs automatically right if you can draw a path selection just in the middle of the anti-alias zone. Feathering a magic wand selection doesn't make it right (tested). With magic wand there's always selected too much or too little.
But you can use the quick selection tool. It's very clever item. You can use it in + or - mode in turns and it seems to learn what you want. Using it in the blue channel gives a good result fastest due the best contrast as you have already noticed. The selection is automatically well anti-aliased.
So, select the shirt in the blue channel with small size quick selection tool and create a new layer. Fill the selection in the new layer with quite dark color and let the layer have blending mode Hard Light. With the color you see clearly the faults in the selection:
There's sharp cusps which aren't selected, so the filled selection doesn't give color to them. You should walk around the selection before any filling and try to fix the selection by using the quick selection tool with small, even 1 px brush size in turns in + and - mode (+Shift, -Alt). I guess it becomes good except sharp cusps.
A piece in the edge of the neck opening is intentionally left bad. There's selected too much. You see something like a bruise in the middle of the edge of the neck opening.
Paint with a small soft brush color to the areas which were left out of the selection. Erase color in areas which seem to be unwanted extras in the selection.
After few strokes with a paintbrush and eraser the new colorization layer is perfect (or as good as resolution this low allows)
I guess you want to have a solid masked colorization layer for the fill color because that can have also patterns. In addition you want to insert an adjustment layer for ex. to increase the contrast of the details in the shirt:
You can use the manually fixed blue layer to make a selection which can be used to create perfect layer masks. Simply Ctrl+Click the layer icon of the previous filled layer in the layers panel. Then create layer masks for the adjustment layer and the colorization layer. I filled a layer with pink for colorization and let that layer have blending mode multiply. The curves layer is used to lift up all wrinkles of the shirt. It must be used carefully because it lifts also up the jagginess.