Today we think of youth much more as “play” but for Browning (or Ms. Barrett at the time of this writing) youth, and especially in relation to what she is speaking of here in regard to her thoughts swirling about in reflection upon Theocritus, meant more time spent in Nature.
Sonnets from the Portuguese was written between 1845 and 1850, and was, in effect, a love letter to Robert Browning. They were married in 1846. She was 40 and died 15 years later. Life was short.
I believe she was feeling that Shape was the force of Nature itself pulling at her which at first she thought was Death, but then realized was love. I am not alone in this opinion.