One of the things the time jump between Heated Rivalry and Long Game robs us of is seeing the effect coming out to his team has on Shane in real time. And that experience does a lot to inspire his anxiety and responses in TLG.
No one will ever convince me that there's not a connection between Shane coming out to his team, it not going well, and Shane volunteering to start an even more intensely restrictive diet than he was already on, that also punishes his relationship. Just like I will always view Shane's anxiety around Ilya coming out to the Centaurs/his friends as a trauma response based on how badly it went for him.
One of the most heartbreaking parts about Shane's performance diet to me, is how in both My Dinner with Hayden and TLG Shane and Ilya mention how they love to cook together, and it's like a fun couple thing for them. It's a point of connection, something they do together aside from hockey. And, unlike hockey, it's something they don't compete about.
Shane's performance diet robs them of this. There's a scene in TLG where Ilya has done a bunch of research to find a meal they can cook together that fits in Shane's diet. And Shane is delighted by this. But, notably, he puts no effort into finding other recipes they can do this with again. He tells Rose he likes the discipline of his diet, and I don't think he means just in regards to what and where he's eating, I think he also means in his relationship.
There's another major change we see in Shane between books. At the end of Heated Rivalry he's talking fairly optimisticly about both of them coming out to friends.
This is at the cottage. Very soon after they've decided they're in a real relationship. It's hard not to compare this with Shane's sheer panic in TLG when Ilya talks about coming out to Troy - not even about them, just about being bi - and also to the possibility of even meeting the rest of his team.
So, what's happened to Shane in the last three years that makes him go from joking around about coming out to his friends to rabidly controlling his relationship and protecting every small detail of it that he possibly can?