Personally, I find this an argument for being more emotionally motivated by near term outcomes (even while being more philosophically motivated by long term outcomes).

Three ways I've seen this done well (often in tandem):

1. Diversity of attention: Working on a longtermist project while paying attention to to how other parts of their lives (e.g. allocating some donations for addressing more immediate problems, changing diet for moral circle expansion, building relationships, hobbies, creative works) bring them joy and motivation. It could be very demotivating if one is purely "all-in" (devoting huge amounts of time/thought/identity) on something that feels incredibly far away.

2. Focus on near-term outcomes of longtermist projects. For example, you're focused on long term political stability and working on changing the voting system of a local government area and the immediate benefit that provides people.

3. Focus on immediate progress towards longtermist projects. Every project has goals that you reach such as getting funding, publishing a paper, hiring a new colleague, running an event. Take the time to celebrate these wins.

I'm afraid I don't have any original recommendations, but have you read the EA Handbook Motivation Series? Nate Soares' 'On Caring' might be particularly relevant.

I was also talking with some other EAs about this recently and one of them mentioned Metta meditation, which is essentially a meditation that focuses on creating an expanding circle of goodwill, which could hypothetically include the long-term future. If meditation is your thing, it might be worth a shot.