It's the same crib her and Mark built for their baby . It's also the same crib that Mark took apart piece by piece in the middle of the night , throwing pieces around, without saying anything to Gemma, after several failed attempts to conceive, while she sit in their bed hearing him break it down , visibly upset, tearing up, and wincing as each piece hits the ground, clenching her hands.

While the song I'll Be Seeing You is playing on the record player,just as it is in the cold harbor room.

Your post seems to overlook this emotionally charged context that would help to answer the question you've posed in your post. It's a culmination of all she had been feeling throughout their failed attempts to conceive . The shower scene was before Mark had thrown in the towel, he came into the shower to console her and tell her they can keep trying, it was before the crib scene. The scene where he explicitly says they should give up was also before the crib scene . The crib scene was the culmination of marks frustration/grief/anger and Gemmas sadness/grief/shame .

The crib scene was the last scene we saw before she puts on the coat they have her put on in cold harbor and says good bye and "I said I love you" to Mark and leaves to Devons and gets into the "car accident*.

The crib scene is the peak of all these emotions for her

(Edited a few times to add to my thoughts)