You are probably a great guy but you don't want to waste your time to think that things will change after she has given you her answer. She may be trying to spare your feelings. Although you can fantasize about her it serves no purpose if she has told you that she doesn't have the same feelings. Women often try to tell someone in a nice way that they aren't interested. The fact that she has done that including the fact that it wouldn't happen if you were both single tells you that she is clearly not wanting the relationship that you want. If you choose to fantasize or harbor these feelings you have to ask yourself for what reason.
I think it is more important that you consider how you will be friends without the hope that you will be a couple. You may be setting yourself up for disappointment. That should be your concern. By continuing to pursue these feelings you will ruin the friendship. You are preventing yourself from finding someone who does have these feelings.
I would accept that you are in the friend zone and that you have to find love in someone who is open to that experience. You deserve to be happy
Please press accept; this is the only way I am compensated