Not all exercise sessions are made equal. If you're just doing 10 pushups or 7 minute circuits for 6 months, you're still very much in the beginner phase.

It took me a few years of consistent intense training to really get into the groove of things. I've been at it for about 9 years now. This is the intensity that I aim for (mind you, this is not me in the video): http://www.youtube.com/watch?v=LAhNyvTF5Uw

After that level of intensity on a consistent basis for a few years, you adapt to it. While you may feel tired, you gain energy from it as well.