blockages

I have had by pass surgery 1 year ago.  One of the grafts has filled to 50%.  I am told that I have blockages in major arteries such as Left Anterior Descending artery, circumflex sp?, arteries behind the heart and branches.  I have diffuse blockage in the LAD plus blockages in several places in the LAD.  They can not stent 3 laterals, because they are too close together and too small.  I am having a lot of angina such as chest squeezing, jaw discomfort, short of breath at times.  My big problem is lately I have had a couple of days (especially after exercise) that I am totally fatigued, and I am taking a lot of nitro.  I also wear the patch.  I just finished rehab and was told there that I have lots of blockages and my symptoms are getting worse.  I am now told by my cardo that I am in danger of having a heart attack.  Do you have any suggestions what a person in my situation should do?  Nitro does help me a lot.  I want to exercise, but I am told not to get my heart rate over 90.  What happens when the grafts fill all the way up.  If one is too small, how do they fix them?  Thanks for the help.