Go to a chiropractor. A recurring migraine for years in the same place could very well be corrected with an adjustment. It sounds to me like you're so clamped down in your vertabrae that blood flow is restricted, and once you start exercising (the muscle warms up), everything opens up...until you stop (and it clamps back down again). That ties into the light headedness and dizzyness as well - all the time. When the neck and shoulders get SO tight like that, it pulls the vertabrae together so that whole area turns into a "brick". That results in pinched nerves, blood pressure/blood flow issues (which turns into dizziness and vision loss)...it's really amazing how it all ties together.
Your asthma doesn't help the situation, since the struggle to breathe just creates further muscle tension in your neck and shoulders. (I have asthma, too. Been on Advair for years.)
I've had a similar problem for a long time. Regular massage and chiro visits have helped IMMENSELY.
PS: your regular doctor won't know how to treat this, so they'll just medicate you. That won't solve the problem. I went to several before I learned that going another route was necessary.