Dull and jealous, I was the smallest part Of the whole. I know this is stupid stuff But I felt less important than the farthest star
The speaker begins here with a bit of humility and honesty. He admits to being "dull", "jealous", and says he was the smallest part of "the whole."
Notice how "dull" comes right after describing his son's mother as being bright as the stars. The contrast works well to emphasize the speaker's feelings. It's like he's giving us a play-by-play of what happened, allowing the repetition and narration to keep us interested.
In line 10, he says he knows what he's talking about is "stupid stuff", which makes him sound sort of sheepish, as if he doesn't really want to admit to feeling "less important than the farthest star." Sounds like he feels far away from the two people he's supposed to love most, and he's a little embarrassed about it.
Why is he telling us all of this anyway? It's not really clear except that we can tell he feels a bit frustrated by what's happening and is describing his discomfort for us. And he can't seem to get away from talking about the stars.
Remember that stars were what he wanted to praise at the beginning? The poem does an awesome job of mixing together what he thought was important at the beginning of the poem with what takes priority for him as a father.
In other words, praising the stars for the speaker has begun to morph into being a good father and husband to his family. The confusing part is that while he wants to help his family, he's realizing there are some things he just can't do as a father, like, oh, breastfeed?