This sounds like a FATCA issue. I will attempt to explain, but please confirm with your own research, as I am not a FATCA expert.
If a foreign institution has made a policy decision not to accept US customers because of the Foreign Financial Institution (FFI) obligations under FATCA, then that will of course exclude you even if you are resident outside the US. The US government asserts the principle of universal tax jurisdiction over its citizens. The institution may have a publicly available FATCA policy statement or otherwise be covered in a new story, so you can confirm this is what has happened. Failing that, I would follow up and ask for clarification.
You may be able to find an institution that accepts US citizens as investors. This requires some research, maybe some legwork.
Renunciation of your citizenship is the most certain way to circumvent this issue, if you are prepared to take such a drastic step. Such a step would require thought and planning. Note that there would be an expatriation tax ("exit tax") that deems a disposition of all your assets (mark to market for all your assets) under IRC § 877.
A less direct but far less extreme measure would be to use an intermediary, either one that has access or a foreign entity (i.e. non-US entity) that can gain access. A Non-Financial Foreign Entity (NFFE) is itself subject to withholding rules of FATCA, so it must withhold payments to you and any other US persons. But the investing institutions will not become FFIs by paying an NFFE; the obligation rests on the FFI.
PWC Australia has a nice little writeup that explains some of the key terms and concepts of FATCA.
Of course, the simplest solution is probably to use US institutions, where possible. Non-foreign entities do not have foreign obligations under FATCA.