It is not your duty to complete the work; neither are you free to desist from it.
Mishna Avot 2.16
This is the strapline of the website of the current Chief Rabbi, Jonathan Sacks. Although I am not Jewish, I think he would not mind my saying I derive a great deal of strength from the two halves of that sentence. To paraphrase: whatever your goal is, do not be discouraged if you think it unlikely to be achieved in your lifetime; irrespective of that, you must continue to struggle to the best of your ability to achieve it.