It really sounds like you have tried it all, and it is obvious you are giving all. I have no answer except to let you know you are not alone out there, and perhaps it is a comfort.
While I have certainly not had a situaiton like this with my son at the extent you describe, we have had and continue to have some very challenging family issues in the past year or so, resulting in super clinginess and crankyness combined with high level of agression at times, almost pushing me over the brink in not being able to meet his emotional need, making me feel I am failing him in some way, not able to alleviate his anxiety.
What does help us is that I whisper to him in his sleep, I tell him that I love him, several times over as he sleeps, and I tell him he is ok the way he is. Especially I do this after particularly horrible evenings or a day when all I feel is drained by his need and i feel used.
It sounds weird but it helps a lot: he visible relaxes in his sleep and the next day he wakes less cranky than usual. I think this way he can actually receive and take in the affirmation that he is ok, and not at fault, and that none of the stuff causing him such anxiety and happening around him is his fault.
The idea comes from a website i found when desperate for help but I don't really agree with a number of the things there, most importantly some of the things one is supposed to say to the sleeping child sound creepy, but the principle itself which is assuring the child of your love works well for us. I also started to make it a point to tell him often when he is awake and in situations I find particularly exasparating that I love him and cuddle him - instead of directly responding to the demand (but only if I can manage sincerity) and it seems to help him be less clingy or agressive. He makes no verbal repsonse but I can see his shoulders relax and jaw unclench. He is older than your child (4 3/4) but I sometimes whish I had done it when he was younger already.
Also, I agree with alphawoman - be careful in the choice of professional if you do seek medical or other professional help, or also if choosing alternative methods. Especially when it comes to bf a toddler, so often bf is blamed for a child's behaviour by those unfamilliar with the concept of extended bf. Children are so easily labelled and it is not always helpful or good. I agree also very much with meg - two years is a young age.
I hope you find some solution which works for you both.