The pain of costochondritis "mimics" a heart attack. I, too, have costochondritis and have been dealing with it off and on for the past year. It will just start out of the blue and it's horrible. I get the pain in my upper left chest that radiates down my left arm, into my upper back and neck. I also have a very tender spot in the upper chest that makes the same pain when I push on it. I can't tell you how many times I almost went to the ER because of it. I know what it is, but when it's happening you can't help but feel worried and panicked.
Some thing that helps me is to take something like an Aleve and then place a heating pad on my upper back and just lie down. Drinking some chamomile tea helps me, too, or a warm bath. It takes about 30 minutes to an hour but eventually I'll feel it start to subside.
My doctor doesn't seem concerned either. I went to see her two weeks ago, actually, about this exact thing. She prescribed a steroid and an anti-inflammatory med but I haven't started either since the pain disappeared. She also suggested that I go and have a stress test done more for my own peace of mind. She knows how anxious I am about it being heart related and thinks if I have the stress test done, it will help to lower my anxiety.
So, I know exactly how you feel and it is really scary. But, try to do something to relax yourself when it happens.