It has taken me years of practice and therapy. Apathy is definitely not a state I can or want to sustain so I try to avoid it too but it creeps up on me. One thing that works for me is to notice the expectation and that takes practice. It is easier to detach when I notice it is just my expectation and not part of some greater truth that someone should be a certain way or do a certain thing because I want them to and have built it up in my own mind, so the world should revolve around me, which was an underlying belief that was influencing my perception. Not saying it is like that for you.
If I feel disappointed by someone now I have no one to blame but myself. I am pretty good at seeing just how people will disappointment me if I don't let go of my expectations. Sometimes it is more wishful thinking on my part that leaves me disappointed and I can see it for what it is and separate that from the person.
Edit: The way I was taught was to pay attention for a whole day and just notice each time I had an expectation, no matter how small. Like in a restaurant. Do I expect to be seated right away? If I am not and I get restless I just remind myself that I expected things to go one way and they didn't. That's life. After a while noticing was more automatic.
If my family/friend/partner does nothing for my birthday or even forgets. I remember I wasn't expecting anything anyway....at least I thought I wasn't considering I made a big deal out of people NOT doing something for my bday or whatever. Why would I let myself feel disappointed after I clearly said I didn't want all that fuss. This is where underlying beliefs influence and where I start to notice. On my bday everyone forgets and I find myself feeling sorry for myself. I still had somewhat of an expectation that if they cared they would do something anyway. Even though I didn't want a party I did want to know they care.
Most people are not mind readers. I tell them to do nothing and they take that at face value. Yet I held the expectation that a caring person would have done it anyway. It is a bit of a mind game. If I want something, first I have to figure out what I really want then state it clearly to others. I can't passively hope they will know what is in my heart and mind if I don't. So really it is about communication and knowing what you want out of your relationships with others.