There are many things in the world that have been done and achieved completely. There are many things that are achieved only partially. There are many things in the world that have not been achieved at all.
People say "Nothing is impossible, we if try sincerely and properly." This is mainly to encourage a person to try to achieve some thing which is difficult. The advice is not to go madly after achieving an impossible task. It is intended for those tasks which are realizable and feasible.
Further, it is perhaps not possible for one person to achieve some task in his/her life time. May be it takes a number of generations for some thing to be achieved. So it means that every feasible task can be made possible.
One point to note - the feasibility of some impossible tasks improves each year, as we are progressing.