I don't think I've seen "sad," "butterflies" "tiger strapped in its cage" clustered together expressing loss of personal love before. It is an interesting spectrum, from blue, to nervous, to a magnificent animal passion, potentially dangerous.
The tiger is also emblematic of awe of creation in the Blake poem, "Tyger, tyger, burning brignt, in the forest of the night/What immortal hand or eye dare frame thy fearful symmetry."
The tone of your poem remains demure, making the most dramatic image all the more strange -- and life is indeed strange, we're all Strangers in a Strange Land.
Second stanza correction: "deepens."
Hmmm, "the butterflies are still there/living inside me/like a tiger strapped in its cage". . .One pauses over that, the butterflies being "like" a tiger. Suggestion: "morphing into" a tiger. . .Or words of your choice expressing the change. "Like" as simile is not only more watered down than a metaphor, but butterflies suddenly being like a tiger is a bit strained. The notion of the metamorphosis though, I think is good, as a butterfly is the result of a metamorphosis from caterpillar. . .and through metaphor may now be a strapped/caged tiger. . .
What impresses me the most about this poem is that its deceptive simplicity stimulated much thought.