So Twelve can’t remember what it felt like to love Clara, or even who she is which has created in a huge void within him. He can’t even mourn her now and he needs to mourn because he hasn’t actually done that yet, he needs to let go but he doesn’t know what of. The Doctor doesn’t like endings and yet insists that there is no way to avoid the end of he and River’s timeline (Darillium). He gives in to it which is categorically unlike him to do so, but deep down he knows it is what he needs. He and River share their last night together, he fills that void for 24 years and then he leaves. He mourns. Unknowingly mourning Clara at the same time.
This all makes perfect sense in my head, probably no one else’s mind you and clear’s up my annoyance at The Return of Doctor Mysterio. Clara isn’t referenced because she doesn’t need to be. We have to keep remembering that Twelve has forgotten his love for Clara. I know there’s all this stuff regarding that Class scene and the portrait on the TARDIS. He might remember that he loved a feisty little lady called Clara, but without remembering who she is how can he still feel it?