I run D&D for a pleasant and attentive group of players on a weekly basis. I have done so for nearly two years. Honestly, they're an excellent group and we have almost no problems, save one: we have never finished a campaign. This is probably because I commit to long modules with world-ending threats, a Game of Thrones cast, huge explorative journeys, and complicated political dynamics. It starts off well, but it quickly becomes so complicated that neither the players or I understand what's happening or where it's going, and it lasts so long that I get bored. So what do I do? I call the campaign to a close and try to learn from my mistakes. 

Then I tried running Tomb of Annihilation. This is an excellent setting with a world-ending threat, a Game of Thrones cast, huge explorative journeys, and many factions engaged in complex politics... you see where this is going. The campaign is becoming convoluted, which is making me feel stressed and bored with the whole affair. Meanwhile, another campaign I'm running for a group of newbies is moving along splendidly. It's about killing a dragon to claim a castle. It's simple, short, and fun. 

Now, I could call my Tomb of Annihilation campaign to a close and start a new, short, simple adventure in the same vein as my second campaign. However, this would be the third time I've done this and I'm concerned my players will have had enough. Simultaneously, I'm becoming so disinterested in this campaign that it's becoming a chore to prepare it. 

Surely I am not the only person who has experienced this problem. What advice do you folks have for me? Should I call it quits and start again? If so, how do I go about ensuring these mistakes are not repeated and people don't want to bite my head off? Or do I commit to my campaign and see it through? If so, how do I keep myself engaged and interested? Thanks for your responses :) 

I would suggest you to maybe try to run a few sessions "on the fly", meaning to just have VERY few notes, just the general plot hook to throw at your players and no more than one or two NPCs sort-of prepared (nothing too deep).
Everything else: improvise on the spot, let the players help you keep the reins and create the story together, make them part of the creative and world-building process on the spot instead of them "just" running the situations you present them with (I am just describing what I mean, I do not assume you do not do any of this), have them come up with an NPC or location description when they are looking for info or exploring or journeying.

It's not easy, at first, but it is an extremely rewarding experience, and it helps everyone feel invested in the story much more than the DM-to-Players usual "mechanic".

Hope this can help you and your friends :) Another option, and a suggestion to you: if you are feeling disinterested and "burned-out", maybe try to see if someone else in the group would like to take up the mantel for one or two short, simple, unrelated adventures. This can help you recharge your batteries and "clean the air" for everyone.