I'm not the OP but I would like to respond. I've tried to fake it a lot but it hasn't worked. But it might be because of my 'self-sabotaging' nature.
Years ago I was in a great relationship with a really beautiful person and the whole time I was also very depressed and suicidal. I tried to feel good and couldn't so I self-sabotaged by telling myself I should leave her so that she can meet someone better. Even though I wasn't doing anything bad at first, I would "purposefully" act like an asshole by being depressed/suicidal. Even during our best moments I would be just completely unable to appreciate it, and think "this isn't making me happy, I should die." Eventually I got my wish and we broke up. Since then I have not tried to pursue a real romantic relationship because I remember that feeling of having it and still not really wanting it any more than death.
About a year later I got a full-time job and my own apartment. I still hated it so much. I remember having friends, doing fun things, and having frequent sex with this really pretty girl. But I still did not want it at all, I just wanted to die. Even during the moments when I WAS SUPPOSED TO FEEL HAPPY (after a long day of work, just got paid, smoking weed in bed with my co-worker after having sex) I still felt that horrible empty feeling of really wanting to die and I even remember her looking at me and asking me what's wrong when I had dazed out, staring at the sun outside the window.
Eventually I quit my job after self-harming the worst I have ever done and since then I've relapsed into what you describe, someone who actively avoids everything and has no motivation. Bc I really had so many great things and I still felt awful, I don't want to have to go through it all again for nothing.
Sorry if I'm hijacking this thread, I would respond privately but maybe someone here relates.