The important thing to remember is that you have been given the "all clear" medically, and that's great! It is VERY hard for people to accept that such REAL physical symptoms could be caused by anxiety, especially in someone like you, who doesn't really feel like they're anxious.
It seems like you've started to accept that this is a psychosomatic problem, which is good. With acceptance, comes results. The best thing for you to do is to start addressing this as an anxiety problem. You'd be surprised, even if you don't FEEL very obviously anxious, you still may be. Everyone's anxiety presents itself differently, some in the form of panic attacks, some just have a constant, generalized worry, and others don't have very obvious anxious symptoms.
My suggestion to you is to ask your doc for a referral to a psychiatrist. He/she can thoroughly assess you, offer a possible diagnosis and cause, and start discussing treatment options, which may include therapy and medications.
It's definitely worth a try, as you've already been dealing with this far too long. The great news is...even if you're still not totally convinced that anxiety is the culprit, when you start facing the anxiety and doing something to address it, the symptoms that were being caused by anxiety will improve. You also may end up being quite shocked at how you end up feeling once you start treatment. Just possibly, you have been anxious all along, and just didn't really recognize it. Down the line, when you compare how you felt starting out...and then after you get into treatment, you may just be amazed at the difference.
It's clear you need to do something, and in my opinion, tackling this as anxiety is as reasonable an approach as any. Best to you, please keep in touch and let us know how you're doing!