if she didn't pay you - it's yours. tell her you plan on selling it to someone else if she doesn't want it. you don't deliver, there is no reason for you to go out of your way to their house, only to be stabbed or something (probably won't happen, but still you should be on home territory. as others said - 50% up front, 25% on approval of design, final 25% when complete, you'll have 75% at that point if they don't pay. and it's still not theirs because they didn't pay yet. i would make a contract for the future that reflects this, along with something that says - the client has XX amount of days to give the final payment or something will happen, i don't know what.
but i would contact her and tell her you can't hold it forever, and that you have another interested client in it, they saw it when they, i dunno, came to buy something else from you. and this is a business, you've waited way too long for it, 3 months is a long time, if she doesn't come up with the money very soon, she won't be able to buy it. see if she falls for that.
---Mike Savad