I don't know about the motivation for exercise part (I struggle with that too - I've only ever found that forcing myself to do it gets me motivated, but that doesn't help when I can't get the motivation to force myself!)
However, I'm a massage therapist, and have some advice for the stiff neck:
1. Take hot showers (not scalding, but very warm). Let the water run down your neck and back for a few minutes to help your muscles relax.
2. While in the shower with the hot water running down your neck, slowly bend your chin to your chest (or as far as possible without causing intense pain). When you get to that point, slowly "walk" your hands down your thighs, bending your back as you go (starting to bend with your shoulders, not just at your waist).
Please MAKE SURE you have solid footing - slipping in the shower wouldn't help any stiffness!
3. A couple times during the day while at your desk, put one hand under your chair (for example, your left hand). Take your right hand and put it on your head bringining your right ear to your right shoulder. Using your hand to control your head movements allows your neck muscles to relax while stretching instead of working while they stretch. Keeping the Left hand under the chair ensures that your left should stays down allowing for a deeper stretch. Repeat on the other side. Hold each stretch for about 30 seconds.
With all of these - only go as far as you can. Don't push your body to intense pain. A little pain is ok, as long as it isn't sharp or shooting pain. But don't over-do it or you'll hurt your muscles worse. If you have any questions or specifics you can always PM me!
Hope that helps!!