It will definitely change. It can still be quite good though, maybe better in some ways, and not even in spite of but rather bc of the differences in your life paths after parting ways physically/geographically. But you aren't married or anything - it has to be fun for the both of you or it will be harder to maintain.

It sounds like that will be harder on you than on him, at least in the short term. But you are engaging in therapy, and willing to think through what is good for him, good for you, good for your continued friendship, and e.g. how that might be a cover / smokescreen for what's really bothering you inside. That's everything that you need to be doing, and you are doing it, even though it's a process and will take a minute to complete.

And I mentioned elsewhere but it seems worth repeating: find other friendships as well, so that you don't pour all of your hopes and energies into this one that isn't as sustainable as when you were geophysically located closer together. This one will be different now than it was before - but it can be good if you want to put in the effort to maintain it, i.e. not hold out unrealistic expectations that it must remain always the same as it used to be, which ofc it literally just cannot ever do.

Yeah, it unfortunately sounds like you might be drifting apart. Hard to say for sure just from reading comments, but that's a thing that happens even though it sucks.

You might get somewhere by asking yourself why this boundary is being put up. It's not explicitly said, but it sounds like this is a new boundary in a longer relationship. Did something happen that caused it? If it's a new boundary, it's probably worth asking your therapist if they can help you figure out a why. More for your own benefit, because even if you figure out a "why", it doesn't sound like it would help the relationship improve.