There is a great deal of wonderful advice here. It is easy to believe that if one does not exit childhood with a set of foundational skills in music or arts or sciences that they have lost their chance to truly achieve skillfulness. If only I had picked up the tuba. If only I drew when it was most interesting. If only I started martial arts when all my gong fu brothers did at six years old. That world will never be. You are here where you are, and first and foremost you must accept that this is your reality.
I'm in my thirties, and honestly there are many days I feel the same as you do, even looking over the great accomplishments I have so far attained, but from the sounds of it, you are still much younger than I. I'll spare you the 'but you're so young!' comments, as I'm sure you get enough of those, but I can relate to you. That lacking feeling of accomplishment, the drive that nets no distance, the nagging feeling that you could have been more. That you are depleted before you even began. Something went wrong. Destiny cannot be fulfilled.
These are reactions. Road blocks. Mental matter.
Step back and look at where you are. Be there. Be here, on this page! Accept that limitations exist, but do not accept them without properly meeting them in person. I have found that often times what I thought were my limits were only foot prints and empty camp sites where the limit once walked. They can often be much further than you'd imagine. I am not discounting your feelings, but I believe as many here have stated, your thoughts of a weak willed self have created a phantasm of who you are, but your will is in truth much stronger than you believe. Nothing we say here can convince you to embody a stronger you, but we can point fingers in the right direction. Ultimately you must conquer your own self, harmonize with your natures of strong and weak, and produce the person you want to be. Some of the strongest people I've known have been weak, and some of the weakest have been strong. What matters is applying yourself in a direction you'd like to go and using the results not to create who you are, but as direction for where you should go next. This is a long winded way to say look at other's advice here and make changes in your life, but do not let the successes and failures of these changes stop you from moving. You may need to grip the steering wheel tightly in the beginning, but eventually you can relax and navigate yourself with greater ease. Take hold, start small. Your end goals of career and partnership will be results of your self direction. Let your will be the judge of your success, not the will of your friends or your family. You must choose. As far as I have decided, that is what it means to be a man, to be a woman, to be an adult.
I think it is easy to come to places like this and assume an energetic deficiency is the root of everything. I know there are many times I have wondered if it was too late for me, that I wont be able to mobilize chi or build a dantien or accomplish anything in alchemy because of my past mistakes, and thus will have an impediment in life. While I cannot diagnose you or even my self as far as esoteric matters go(I do not yet have the adequate knowledge to speak with authority here), perhaps I can suggest something a bit more secular. Let us say that worst case, you are depleted of your life essences. You are deficient. It is a fundamental flaw due to old habits and bad behavior. You are doomed to a life of weakness. Can you will yourself to be strong in the face of physical and mental weakness? Will you will yourself to be strong and act with a feeble body? What is the alternative, watch TV until the end? Everyone wants to feel sharp and on top of their game. Why else would we try strange herbal formulas and commit ourselves to odd schedules and exercise routines? But is a strong will not one that would accept that weakness is a part of life, that weakness comes and goes, and realize that one can operate regardless of it? That one can accomplish regardless of their current state, given the right view? Is that not the function of a strong will, to overcome adversity?
I don't know enough about the side of cultivation to tell you if there is some point you are drained too much and locked out of any true progress, if someone can just be too weak or drained or sick to realize 'Truth' (actually would like to know how people feel about this idea). I don't know if you are afraid of some kind of timeline that you need to be at in order to be successful in life. I think sometimes we can get caught up in the end goals like a degree or a family, or trapped in words like jing and chi, and forget the incredible intricacies that these terms and dreams are made of. A million tiny successes and a million tiny errors make up fulfilled dreams. The energies in the Daoist system create the experimental reality of existence itself, filled with the unending minutia of color and vibrancy. We may be able to distill it down to "yer burning too much off them there kidneys, son" but don't think for a second that this makes you unable to live, even if that is what we consider the literal definition of leaking jing. Call it ignorance on my part, but don't blindly give in to the notion that you've weakened yourself and thus you are unable to exert authority over yourself, that you are unable to become who you want to be, that you are somehow moving against nature and thus unable to grow or even adapt.
If you are reading this you are not too old to breathe, and if you can breathe, you can appreciate who you are right now. Can you will yourself to see brightness in your heart? If you decide it is too late for you, then it will be too late. If it really is too late, then clean your desk. Notice you had the unparalleled power to do so. Do a push up, just one. Okay, now give me two! Notice the strength it took to do so. Find a real dope tree and practice qigong beside it, notice how even in perfect relaxation you assert yourself towards your goals. Complete your classwork, revel in the fact that you didn't just do it because you had to, or because you have had some previous predilection towards educational ease, but that you sat yourself down and accomplished a task of your own will. Make a change to drink more water, to start heading to bed earlier, to smile at an attractive woman. Begin with what you can do today. The will you use to start the small tasks is the same will you use to complete the big ones. Change is infinite. Our paths are never straight, yet they always head in one direction. When these feelings return, and they will return, face them, accept them, and then go do something meaningful.
Even writing glib platitudes can be the willful practice of an aspiring writer.