Ah! So that's what I've been doing wrong. When I tried to go to the gym regularly with the goal of getting stronger/bigger/having more energy, the actual process of exercising was merely instrumental to me so I couldn't motivate myself to do it consistently. Two of my friends who are more successful at exercising than me have confirmed that for them exercising is both instrumental and a goal in and of itself.

But while I'm down with the idea of hacking terminal goals, I have no idea how to do that. Whereas compartmentalizing is easy (just ignore evidence against the position you want to believe), goal hacking sounds very difficult. Any suggestions/resources for learning how to do this?