"1: It's not how big it is, it's how you use it."
First off, don't buy your entire 2000 points all at once. Not only is this sensible in practical terms (no point in spending $500 on something that you play a few times and decide you don't like very much, is there?) but it's altogether easier to put together, too.
I have my own private room set aside for my hobbies, and I *still* trip over the piles of stuff that I'm working on. Less clutter makes it easier to work on your current project, and the less stuff you have that you want to paint, the easier it'll be to get around to the stuff you need
1a: "Whadda you mean, 'you didn't read rule #1?' ya bozo!?"
Oh well. No worries. It happens to the best of us. When I finish painting my Blood Bowl Teams (plural) and my 2500 point Fantasy Army (all purchased circa. 2003....) maybe then I'll criticise you for making the same mistake.
In this case, pick out a few boxes of stuff - a HQ, 2 Troops and maybe a couple of transports or Heavy - and stuff the rest under the bed. Or in a cupboard, or the attic, or something. Get them out of sight, so that you don't have to worry about them, and then forget about them until later. Congratulations - you now only have a 500 points army to paint. Doesn't that feel much better?
2: "Know Your Limits"
I find that variety helps you refrain from insanity. Take one Squad and put it together - ignore all the rest for now. Do as much painting of them as you like until you find your attention starting to flag, and then glue up something else, like a vehicle or character.
A change is as good as a rest, and after losing a few more fingerprints to the superglue (if you're anything like me, at least...) you'll feel better about painting again.
One squad and one vehicle on the go at a time, so you're always working on SOMETHING but never have an enormous pile of stuff that you're trying to grind your way through. Nothing is more demoralising to a painter than seeing a box full of stuff that you have to grind your way through, but if you can trick yourself into thinking of it as several different, smaller tasks - one squad to assemble, one to spray, one to basecoat, another to varnish - you'll find yourself appreciating the little achievements every so often.
Remember also: It's not a race to the finish. The only people who paint armies to deadlines are the guys who try to make a living off'f doing it on other peoples' behalf. We're doing this for a hobby - we're trying to kill as much time as we can doing something that we enjoy, not to blitz through it as fast as possible and into the luke-warm, languid pools of boredom awaiting on the other side!
3: "You want that Super-Powered Unit of Game-Destroying Destrucity? Earn it, maggot!"
If nothing else, don't make the mistake that I usually do of assembling and painting all your favourite stuff first.
Yes, that Warlord and his enormous converted Battlewagon look fantastic, but now you have nothing to look forward to until you've finished those 60 near-identical Boyz, AND you can't use them to play smaller games while the rest of your army is still on the go.
Squad by squad, vehicle by vehicle, you'll at least have a legal 500 (or whatever) points army, which will give you something to enjoy while you're taking a break from painting. Treat your big shiny Tanks/Monstrous Creatures/Whatever as a reward for hammering out the necessary Core units of your army - not only will it give you something to focus on while weathering the tedium of 50 Genestealers, but when you get to paint something you really want to paint it'll help reinvigorate your enthusiasm and give you a boost for moving on to the next stage.
4: "No matter what, you're only human."
The rest is just discipline and patience. It's an unfortunate, but true, fact that some people don't have the patience or time to build up a 40k army.
That's okay. Really, it is.
Go back to #2 and decide from the beginning if you want a perfect, elaborate and highly professional looking army, or if you'd rather just throw stuff together quickly and spend the time you saved playing the game. Plan your army and buy accordingly, or you'll only find yourself demoralised if you line up your finished squads and see them get messier and messier as your interest wanes.
At the end of the day, we're just playing with toy soldiers. If you aren't enjoying it, then you don't have to do it.
5: "No matter how bad you have it, someone else has it worse. Be proud that you're not them."
Every game store has 'That Guy'.
No, not the Rules Lawyer, or the Power Gamer. Though they're usually there too (and in my experience are often the same person) I'm talking about The Guy who turns up to every game with a box full of mangled plastic where his army should be.
I'm sure you know him. He's had it for years, but the most he's done is badly spray his stuff in (usually different) undercoats. His poor, long-suffering troops are woefully unarmed, in terms of both WYSIWYG weaponry and actual limbs. Some of them are just a pair of legs glued lopsidedly to an old base. Some of them are just an old base - having the word 'Vet. Sjt.' [sic] scrawled onto it in Sharpie is optional.
And all he does is complain that he has so much to do that none of it ever gets done. Baw.
Don't be that guy.
Be proud of the fact that you're not him. Fair enough, at some point or another you're going to turn up to a game with The Black Darkeners Chapter of Space Marines - we all do. I've even been know to voluntarily take casualties as my newly-glued models melt into sad piles of limbs on the table and need rescuing before they completely dry like that.
And you know what? No one minds. So long as you have something to show off before the next game - something that you have achieved, something that brings you even a little bit closer to the ultimate goal of a finished, immaculate army - then you're already one step ahead of a reasonably large percentage of people who play the game.
And of course, as we all, the more paint you have on your army then the more likely you are to win games. Just ask any Ork Player you know and he'll tell you that the red ones are always preferable to grey, and any Ultramarines Player will be more than happy to point out how many different colours you'll find in the successor chapters to Robot Girlyman.