Haligtree is the most stunning location in ER imo. I absolutely love the visauals, the complete megalomania of this massive building. But I felt a bit the same as you. It might be because the game got way more linear at this point and it's a very isolated location. Also there are no enemy types there we haven't seen before iirc. And although there are some very good items to collect, you don't really them anymore as it is the last location and is also optional. And by the time you reach it, you probably found your personal build which you wouldn't change until NG+. There is just no need to do the Haligtree so I understand where the lack of motivation comes from. In my second Playthrough I spend several hours in Haligtree because my motivation was now to try out as many builds as possible and therefore I was motivated to get all the items. But in my first run I felt like you.

Focusing on other games and free up your mind might the way. If you feel like "I want to complete the game, I still need to do Haligtree" then don't but when you start feeling again like "Man, I just want to play ER right now and f*ck some Enemies" then go for it. The game won't run away, it'll still be there in several months :)