Perhaps it's just the companies you're working for. All the companies I worked for were pretty relaxed about schedules and I always ended up getting stuff finished way ahead of time.

If you're working too long hours and getting stressed out about it, it's perfectly normal to have attention and concentration problems. Perhaps you just need to relax a bit more, take breaks, and you might find you get more done. Also don't work for dipshit companies that put too much pressure on you!