How to find motivation while working on a long project?

Motivation gets your foot through the door, routine is what keeps you on track. Establish a sustainable routine - set certain days and times of the week you can adhere to 100%, even if it's just 15 minutes. Follow through for a couple of months until it becomes more habitual. Don't rely solely on motivation.

Additionally, make sure you know exactly what the task is every day, so you don't end up wasting time.

Set small goals and even smaller tasks. I use hack n plan to create tickets for all small tasks. That way I see every little bit of progress.

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I just imagine how the final product is gonna end up and gets me excited

It can be difficult to work on long projects (on your own) and stay motivated. I like to create some small side projects from time to time before heading back to the primary project. For this, I like to join small game jams.

Furthermore, you should start a Trello board. This helps keeping track of what is done, and what still needs to be done. It also motivates me a lot when looking at how much progress I have made.

Perhaps only one part of the puzzle with motivation, but I'd ask myself if the game's concept is strong enough or meaningful enough or gets you excited enough. If you're really excited by the idea for the game and/or loving playing it yourself, the motivation should take care of itself. You might also make sure you're aligned with the work itself - if you love making art but your game is code/systems heavy you probably won't look forward to working on it.

Have you tried not giving up half way through?

Probably not the answer you wanted to hear but I think going through a phase of lacking motivation is quite normal. I am working on my game for 3+ years and I completely lost motivation for at least 6 months. Then it slowly came back.

I also permanently have doubts about my game and even hate it sometimes. Just accept these phases, do something else and the movitation will come back eventually.

What helped me most was to properly prototype. I try to create a quick prototype with relevant mechanics that i think are important for the game to be fun. If I can't create this prototype in 1 month the project is over scoped (that's my rule of thumb). I play around with the prototype and really try to find something fun about it that I genuinely want to spend time with. This is very important for me because half way through a project I can't really judge if the game is fun anymore, but i know its fun because I tested it in the beginning. When I lack motivation I try to do stuff on the game that i really like even if it's not too of the priority list at the moment. For example I love creating small interactions and cools shader effects so I might implement something like "when the player is close to a poisonous plant the vision will be distorted and over saturated and the character says a funny line as if he is high". This stuff usually doesn't take long and doesn't affect many parts of the game, perfect to get back into it.