Working feverishly, he built his house in his twenties and built his tomb in his thirties.
夢中 is crazy about. So, if I (talking about you here) am nothing when put next to this thing you're crazy about, then you are really, really into it. You have become nothing, at least when you compare yourself to it. This is total absorption (though it can also be ecstasy).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.