How do you keep focused on a long-term academic goal while working?
You're right: it is easy to get sidetracked by the immediate responsibilities of work and other parts of life.  The first question that I would recommend you ask yourself is: why is that a bad thing?

A higher degree ambition is a great thing, and can lead to a very fulfilling life.  But you can also have a very fulfilling life in a lot of other ways as well.  What would happen if you just let go of the ambition and abandoned the plan?  Would that be so bad?

If it would be so bad, then there must be a reason why.  Now, that's the driver for your long-term goal.  The problem is, how do you get from where you are to that long-term goal, when all of these distractions are around you?  Some of the best advice that I have heard on this subject comes from writing podcast that I enjoy listening to: in one episode the podcasters, who are all well-established fiction writers, were talking about the shifts they had made on their path from aspiring writer to well-paid professionals.  One big shift that they all identified was that they had had to start thinking of writing as their job even while they were not yet being paid for it, which meant setting aside significant amounts of time per day to work on writing.

I would recommend the same thing for you in terms of making sure that you keep your focus.  Set aside a weekly schedule of time to work on the things that you need to do in order to be able to gain admission to the program you are aiming at and to prepare yourself academically for it.  With that time, you can both organize yourself, planning the short term goals that will move you toward your long-term goal, and also you can work on those steps.  

Then, keep records of how much time you are actually devoting toward your academic goals.  If it's much less than you intend, then you need to reassess your priorities and ask again: do you really want this degree if you are not setting aside time to prepare for it?  And if you do really want it, what are you going to de-prioritize in order to make the time you need?

Preparing for higher education while working a full-time job can be hard, but it's also very possible, and I know many people who have done it.