Tristan

I am trying to find a source of musical analysis of Tristan und Isolde as I am not clear on why the unresolved chord in the Prelude remains unesolved for 4 hours. (33 measures before Act II scene 3, for example). It seems to me the composer abandoned the unresolution for most of the work and moved on to other things. Put another way, if the first 18 measures didn't exist, how would the rest of the 3 hours and 59 minutes be any different?