You may have grounds to get your plea vacated if you can show that the coersion was of the type that kept you from entering a plea knowingly and voluntarily. The standard is quite high, however, and the mere fact that your attorney advised you of a dim trial outlook-- or that the evidence against you was powerful-- will generally not be enough.
An important issue to consider is whether you would want to be successful in such a motion. If successful, you would be subject to re prosecution and would find yourself in the same position that caused you accept the plea in the first place. Only this time it is likely that the same plea offer won't be on the table.
As we don't know enough about your case, you should not consider my answer to be legal advice. You would need to consult with an attorney at length and include details about what has occurred in order to adequately assess your criminal exposure.
The short answer is that you file a motion to withdraw your plea. The motion would be to the original court that took the plea. You do not need an attorney that specializes in appellate work as any competent criminal defense attorney should be able to assist you. Whether the motion is sucessful, depends upon the facts of the case. But you should know that these types of motions are very difficult to win. When you plea, the judge asks you a whole series of questions which are designed to prevent you from making these types of arguments at a later date. So you are facing an uphill battle. But there really is no downside to filing the motion, since if it is not successful all that will happen is that you will be in no worse position then you currently are.
If you would like to discuss your case further, please feel free to give me a call.