on drawing III

I cannot draw the pink cup; it is too difficult. I draw for about two minutes, producing a tentative, disproportioned sketch on the back of an envelope. But my heart is not in the work, I do not wish to draw the cup, and confronted by the task, am forced up against my own limitations and discouraged. I lose hope, and shed tears.
I am obliged to rethink my practice. What do I expect from a drawing? What defines a successful drawing to me? I know that a drawing can be as simple as two lines crossed at intervals by shorter lines, made in chalk on a garden path. I find myself wondering if I could not reproduce those lines in chalk, on paper, or, severed from their context would they still exist as successful drawings? But context was not the sole reason why the lines were successful as drawings in their own right; some sections of our childish railway had a genuine aesthetic presence independent from their purpose. However, I cannot reproduce the relationship of chalk to concrete, on paper, neither can I replicate the proportions of the lines to the path. Would it be possible to make fragile lines in pencil echoing those earlier chalk lines, on paper ? I am afraid to make the attempt.
I feel that I need an object to draw that can funtion symbolically, but I may be mistaken in feeling this. At present I cannot trust my feelings.
My work is not abstract, although it cannot be described as purely figurative, or representational. I do not deal with the problems of composition; in my drawings, the isolation of a figure in an expanse of white paper suggests a struggle with context and composition that I have not come to terms with, and instead avoid, preferring to deal instead with lack of context. I know that my powers of draughting are limited; I have never been able to draw with any great skill, my attempts at life drawing when at Art School, for example, were very poor. I do not have the eye for proportion, or the ability to transfer what I do see to the page. I have a poor comprehension of spatial relationships.
Yet still remains this stubborness to persist in making art, a desire to make drawings that amounts to a need. What is to be done?