.... I'm really bad at pep talks, so I hope this doesn't go down the wrong way.
1) you SHOULD be in it for the fun of it. For the "look what I can do with my own two hands" wow feeling. As many before me state : to hell with time limits. If it takes you 10 years to complete it, so be it. Just find a way to have FUN while working on it. The moment you start seeing it as a chore, just walk away for some time and come back to it when you're ready for it. October you say ? Fine. You didn't specifiy the year though.

If what you want to do is just too high to attain, switch back gear. Make a simpler design which you can use to come back later and add to, to obtain your ultimate goal later. Example : I did promise my son a full H3 or H4 armor, but there's no way I'll be able to build a full one this year for him. So next con, he'll go as a H4/H5 Cloacked Spartan, he'll have that cool helmet and rifle, the gauntlets, the boots and the shoulder pieces. And next year, it's time to strip away that cloak when I've produced the rest of his armour by then. Because just sitting in a corner trying to THINK the armor together won't really help. You've got to start somewhere, and best is to aim for a goal that you KNOW you can pull off if you're hellbend on hitting a target within a specified time frame.
2) i KNOW it can be very discouraging to see how good many of the guys&girls on the 405th are at their thing. If I compare my suit to many of them, mine looks like total crap. But did I had fun making it, altering it, repairing it, changing it, sometimes ruining it ? You bet ya. So do NOT stare yourself blind on what other people can do in terms of quality. It is YOUR BUILD, and the only thing that is important is that YOU have a good time working on it, learning to do stuff you've never done before and seeing how you can get BETTER at it than you ever thought was possible. As said, I KNOW my armor is only so-and-so and you know what ? The first kid that walks to you at any con with a wide eye look asking for a picture, and getting over the moon when he gets that High-Five from that Mr Spartan or gets to hold that big$ss gun for his pose with you, THAT is what you do it for. All the rest, to me at least, is BS.
For me, the "OMG" moment was when a young guy in a wheel chair poked my knee (approx), I turned around and I didn't immideatly see him as it was pretty crowdy and I focused on "normal height". Anyway, the SMILE on that guys' face when "the spartan" didn't only let his picture being taken, but even posed with him and his dog for a second one didn't make my day, it made my year. It's a good thing he couldn't see my face through the visor as I had diffculties keeping my eyes dry at that moment.
You want motivation ? THAT's you motivation. Putting a smile on other peoples' faces. If that isn't enough -and I understand it won't be for other people-, I can't help you.
For me, that is more than enough to forget all the hotglue burns, the frustrations when something just does NOT work out, the knowledge the suit from X, the paintjob done by Y and the weaponary done by Z looks a lot better than anything I could cobble together TODAY but who knows maybe someday, the setbacks when that model your worked on so long and so hard goes to ***** in front of your very eyes, the bloody much-higher-than-you-anticipated bill that damn suit has mounted up already and it's still not near ready.
And utterly, I did it because I promised my kids I would do so. From that point on, there's no turning back.
Find that thing that makes YOU refuse to turn back, and just get going.