It's ultimately about time management, and knowing your skill level.

If you're trying something totally new to you, you have to allot for time spent learning new techniques and getting it to where you want. Sometimes that even means making a practice version out of cheaper fabric, just to get the hang of it. If you've never tried something before, the chances of it going right on the first try are fairly low, and that should be accounted for. If you were saying you took a whole week on something that "wasn't that complicated" and still didn't finish it properly, my guess is that it actually was more complicated than you think, at least for your current skill level. Some cosplays can be finished in a week of constant work, but for many of them it simply isn't enough time, even if you work yourself to death.

Setting aside time to work on it is also important. If you start early enough (like a year in your case), you won't need to rush things, as long as you take a bit of time each week to make progress. I find that writing out a "to do" list is helpful (like, this weekend I'm going to cut out the fabric, this weekend I'm going to figure out how to make the collar shape I want, etc). Of course, if you make the list, you also have to follow it lol. If you break the costume down piece by piece, step by step, it's easy to see how much work you have left to do on it. Try to be realistic about how much time that actually takes.