The best way to stay motivated with a fort is to set out with a specific goal in mind.
My most recent fort, I use the masterwork mod. My goal was to 1) attract the king, 2) make sure he had children, and 3) develop a noble and glorious lineage of royal blood. My king and all 3 of his sons are legendary swordsdwarves. I made for him a suit of masterwork gold armor inlaid with emeralds, sapphires, and rubies for official, state-sponsored events (such as the death of my fortresses' greatest warrior Nish Poisonarrows. He died single-handedly holding off an entire Pandashi siege so the civilians could escape. I gave him a traditional dwarven burial, i.e. bury him in a platinum sarcophagus, have the king in his ornamental blingin' armor "give a speech to the crowd" basically using lots of burrows so my entire fortress was present, then dumped his tomb into the magma sea via lever which the king himself pulled. Was... well, I thought it was pretty epic anyway). For battle, the king has a suit of candy. When he inevitably dies due to some dwarven stupidity, his armor will be passed down to his eldest son. I'm rather happy with how this fort is going so far.
I'm currently deciding on a design for a new vast Cathedral to Armok made entirely of marble blocks, silver blocks, and gold blocks. It will be on a hill over a river. Inside the chapel will be waterfalls, magmafalls, and somehow by Armok I'm going to get a pool of blood at the altar. Maybe dump a sock into it too. Beneath the cathedral will be storage, scriptoriums, cells for the "monks" living there, etc. They will grow red and white grapes and export only wine and books. Migrants I will title "pilgrims" who will either earn their initiation into the Cathedral or be left outside in the small village at the foot of the hill. I haven't found a good looking site yet though, I've already genned like 10 worlds -_-
That's what I mean though, before you even embark, have a plan ready to implement and do whatever it takes to get that plan done.