Having read through all of the posts you guys made here, I would have to say that, in having the Sun and Mercury in the 12th house, I don't necessarily feel the deep longing, or boredom you guys describe here.
I understand, all too well the feelings of boredom, as well as the sensation of existing but, not living. It's just that, this feeling isn't one that has ever affected me adversely and it is probably due to having the Moon(Feelings) trine(of) Sun(Happiness).
The 12th house is the house of transcendence, ascension, divinity, immateriality, oneness, spirituality, and dissolution, as well as emptiness, apathy, shapelessness, nothingness, and meaninglessness. These themes, although occasionally experienced by most, are daily realities for those with planets posited in the 12th house( in,primarily, adverse aspects).
The Sun represents our ability to enjoy life, have fun, and live purposefully(willfully) and in the 12th house, the ability to enjoy or have fun goes well beyond who we are as individuals and is supposed to be used to help *others* experience joy and happiness, which can therefore cause us to *personally* feel unacknowledged, invisible, and in many ways, bored.
The 12th house is a universal house which means that it's focus isn't on anything or anyone in particular. The things we experience in this house not only takes the focus off of the subjective self, but being that the 12th is associated with the water element, this house connects us to things that are invisible, intangible (such as dreams, fantasies, spirits, psychic visions, feelings, and emotions),and beyond physical perspective.
The Sun also represents purpose and in the 12th house it is apart of our purpose to transcend our egos and the will to endeavor in, only, our personal enjoyment. We must learn to shine the light and attention on all beings and possibly even spiritual beings such as the God of our understanding and the universe for being who they are.
If we don't learn to acknowledge compassion, love, humility, acceptance, and unity then we will feel trapped within this physical realm and truly experience the confinement and institution of the soul.
We must realize that happiness exists within and beyond this world and believe that sometimes a feeling of nothingness is really just contentment and peace.
We don't always need to "do" something in order to feel fulfilled.
Fulfillment is always there for us to tap into, we simply need to open ourselves to it.
Also, to answer your question I believe that you should look to the signs you have the Sun,Venus,Mercury,Jupiter and even the 12th house cusp placed in, in order to ascertain what type of activities you should endeavor in, to help give you a sense of purpose and a reason to live.
Remember, life is purpose.
Being 20 years old, I can relate to these feelings deeply, in that I experienced them just a few years ago but, somehow they solved themselves when I allowed myself to just care about people and to stop focusing on myself all the time and to acknowledge everyone Including myself.
I wish you good luck buddy and how you get something from this.