Making a character doomed by the narrative interesting?

I need to make a character doomed to die interesting, basically. This character is set up to hunt down his wife’s murderer and me and the DM have planned for him to sacrifice himself to get said kill. However, this character before that point is just really BORING to play, and I’m struggling to be interesting as a result. I initially planned to kill him early and didn’t consider these things all that much, but the plot has evolved and now his death is a lot further out, so I’m spending entire sessions having no personality and it’s killing me.

I mistakenly wrote the character to totally BE his backstory, and not to make an interesting character first, so he’s painfully boring in most situations (think a generic Western/cowboy type, he’s not much beyond that as a character with traits or anything). He’s nothing more than his brooding backstory, so I’m struggling to enjoy the kinda goofy campaign as a result. Due to these plot constraints, I don’t want to jump ship on characters yet to finish my narrative and to not disappoint the DM.

How do I make someone like this interesting without sacrificing his entire character? I know a flashback/dream regarding his wife will be played out soon, so I’ve considered a mental break/realignment of ideals after said scene could be interesting, but how would one play that out without just being mean to other party members?

Any advice is appreciated, thanks all.