Interesting question; let me give you my interpretation on the whole "true self" thing :
To put it simply, i just cut it into two easy to digest chunks; you have your awareness/consciousness and your mind.
Your mind/ego is basically all of your thoughts and feelings. Your consciousness/awareness (whatever word works best), is the "true" you. Your mind is kind of like an interpreter for your consciousness, an advisor of sorts, it acts as a lens you view the world through; your consciousness makes the decisions based on what is provided by this lens. A visual metaphor could be that you are the eyes, and the glasses you wear over your eyes are the mind.
Everyones "eye" may be the same, it is only the "lens" in the glasses that differ. The lens changes, and reshapes as you undergo new experiences, but the eye remains the same, merely watching, and making decisions.
There are two ways i can see that i would attack the problems you're having regarding motivation :
1. The problem is that you think there is a problem. To clarify, the feeling that you are not being your true self (a feeling that is coming in through your lens) is what is causing you to stop doing what it is you're doing. How do you fix it? Simply realize you're not your mind, and act despite the feeling your mind is throwing up at you..Easier said than done ofcourse, but it is common for people who strive to be the best in certain fields that they come to hate their own subject; but it is being able to ignore that feeling and keep going that cuts the wheat from the chaff. The more you resist your minds desires, the better you'll get at resisting it; which ultimately means that at the start of the resistance it will always be the hardest; but once you're over that hump it is smooth sailing, as long as you don't regress back to old habits.
2. If you're setting goals externally, it likely means that you're extracting your validation externally also. This often goes hand in hand with a mindset that is outcome oriented. Both of these ways of thinking aren't necessarily wrong, but they do often come with a lot of suffering. Being outcome oriented means you need a certain outcome to be satisfied, if the desired outcome does not occur, you usually feel suffering of some sort depending on how emotionally invested you are in the outcome. If you are not outcome oriented, but instead process oriented, your goal there is merely choosing to do the process as opposed to desiring the outcome. This comes with no suffering because everything is within your control; just like the difference between intrinsic, and extrinsic motivation. Intrinsic motivation is wanting something that is within your control (Process-oriented); Extrinsic is wanting something that is outside of your control (outcome oriented). If you want to be intrinsically motivated, merely count success as choosing to do the process, rather than being focused on the result of the process. An example :
Extrinsic : Wanting to make someone laugh (goal outside of your control)
Intrinsic : Wanting to tell a joke (goal inside of your control)
Extrinsic : Wanting your crush to like you (goal outside of your control)
Intrinsic : wanting to talk to your crush (goal within your control)
Extrinsic : Wanting to write a best selling novel (outside control)
Intrinsic : Wanting to write (inside control)
So basically, forget about the outcome, focus on the process. How do you do that? Well, i'd start by having an awareness over what your thoughts are saying, and just to ignore them if they don't align with your decided goal. Your mind can be your worst enemy, and it is not you; treat it as a separate body. You want to control your mind, not have your mind controlling you; and you do that simply by consciously being aware of the thoughts and feelings your mind throws your way. Once you're aware of your thoughts and feelings you can then actively go against them as opposed to being on autopilot and letting them run you.
But yes.. Thats just my take on the whole thing, hope it helped.. Went on a bit longer than i'd expected.