Ah, mid game fatigue. I think almost every Dwarf Fortress player felt this way at least once. For some it's a chronic condition, though.
The biggest culprits...
Architecture mishaps: after having built/dug out most of it, you discover you don't like the layout of your fortress.
Defenses not working as intended: ambushes almost intelligently waiting outside in front of your entrance, instead of attempting to cross your fully trapped hall.
Poor stockpile management: you have hoarding tendencies, and at the same time you hate huge open stockpiles. For some reason you hate temporary stockpiles as well. Cue unending quantum stockpiles micromanagement.
The drudgery of trading: even if trading is not that much of an hassle, sometimes it feels like it, and you wish merchants just stopped coming for a year or two.
The 'goblin recycling plant syndrome': killing goblins is awesome and everything, but after some years your whole fort devolves into a goblin processing plant, and not even the free stuff you get is worth anymore the free time you lose cleaning after every ambush and siege. And you can't stop cleaning either, for the sake of you FPS.
Things you can do:
-Be bold! Try new solutions to old problems. Do something you've never done before, something the very thought is alien to you; the forum will gladly supply you with ideas. If you end up not liking it, you were gonna abandon that fort anyway; at least you learned something. It's usually the 'joke fortresses' with nothing to lose that end up being the most interesting and memorable, and sometimes as the most successful as well.
-Kinda the opposite from above: plan the final architecture and appearance from the very beginning of the fortress, even if it means spending an hour assigning digging designations, strategically blocked from time to time to not overwork your miners (if you're gonna build an above ground fort, this is gonna be a bit different).
-Smaller populations are much more manageable. After even as little as 40, dwarves become numbers more than individuals you know and remember. This is gonna be a problem if you like sieges though, unless you play Fortress Defense.
-Experiment with defense design. Sooner or later you'll find your own favorite system.
-Take a sabbatical year from trading, if you find it stressful. Even if you're sorely tempted to just trade them a couple of stacks of masterful roasts and you keep telling yourself it's minimal work for something your dwarves may need one day, and you feel it like a huge waste to let them go away. Merchants are your garbage collectors, not the other way around.
-Fiddle with the advanced world generation parameters. Give yourself 10 z-levels before the first cavern, lower the titan attack wealth and population requirement.
-If you're not averse to mod editing, edit the game to suit your demands. Example: settle in the badlands and start a Giant Desert Scorpion farm. Overpowered? Yes, but damn Fun.
-If you keep losing interest and restarting anyway, don't fight it for a while. Refine your early game techniques and embarking strategies. Before the eighteenth 2 years old fort or so you should get the inspiration, or an idea for a project, to pull through the dreaded mid game.
-Try some succession games, and, if possible, ask for your turn to be on the fourth year. Unless you can't stand succession games, of course.
If all of this fails, it's not necessarily a bad thing. Every fort is a new lesson
