I want to say how much your volunteering work and giving blood every two months are valuable, and suggest you also volunteer in positions that involve children, such as tutoring help via the public library - this way you would start fulfilling your purpose of mattering in a child's life right away.
I think you need to distract yourself and get busy doing things that matter a lot and then it will be easier to attract the right man, possibly via online dating. You are probably wearing the mask of frustration and disappointment on your face daily and that pushes men who otherwise would be prospects far, far away.