I have been on a diet for the better-or worse-part of my lif

Usually this is said,"for better or (for) worse" (without the).This phrase is used to indicate that a fact you are mentioning is not necessarily a good thing or bad thing.Often it is used if you say something,and you do not want to imply that you think it is a good thing.
e.g.Nowadays,anybody can easily publish articles on the Internet,for better or worse.
This is used sometimes like:"for the better or worse part [of my life]",which would be "more than half my lifetime,but using better is somewhat misleading,as the time really was not good,but bad".
自己理解一下……
这里面主要用better引出worse 语境强调的是worse