Rather than filling in the void, I think it ends up doing quite the opposite... You make the empty place even larger, more empty and needy. The more you think about it without some form of satisfaction, the more crave it. Not only does this make you want more, it makes you think that the object of your fantasy actually is more important than it ever ought to have been. Like, you've just spent 10 years obsessing over a woman. You might end up thinking that she's actually your entire purpose for living. You did just devote ten years to her after all! She must be important, really important, something to stake your life on maybe! After a while you may forget that there are other options. Having thought about one person for so long, it's hard to imagine any alternatives. This is unhealthy thinking that cannot possibly lead to anything good.
I think that it's best to find an outlet for your desires rather than bottling them up. If something is truly beyond your reach, then maybe you should find something else to take its stead. Nobody should be so important that your only source of gratification is fantasizing about them. While fantasies are fine, you also deserve to go out and experience real love from another living, breathing human.