Doing My Best,
We are always moving in one direction through spacetime. The speed we move at is a constant 186,000 miles per second. It’s like your in a car that is stuck moving forward at a constant speed of 186,000 miles per second. This car has no break or reverse option, and the steering wheel won’t allow you to make a 360.
When due to inertia you are sitting still in the spatial dimensions all of your movement is through the time dimension. If you start moving in the spatial dimensions you pass through less time because some of your speed is diverted to the the spatial dimensions you are moving in (time passes slower for you). This is why light or any other massless thing must move through space at 186,000 miles per second. There’s nothing holding it still in any of the spatial dimensions.
When you move in one of the spatial dimensions It’s like you turned your car northeast as opposed to straight north. Your still moving forward but you will now cover less ground in the northern direction then a car going straight north. You can’t slow down or speed up this movement through spacetime. You also can’t change the direction of this movement. You can move through more of one of the dimensions or another, in the same way you can angle the car in my example a bit east but your direction in spacetime is always a straight line forward.