You are correct, you need or needed an attorney. In the US, nobody is ever forced to plead anything, much less guilty. If you refuse to enter a plea, the court will enter a plea of "not guilty" on your behalf. Your description of the events doesn't match what is possible under US law.
When you are charged with a crime, the prosecution might negotiate an agreement (plea bargain) with you to the effect that you will plead guilty to X in exchange for Y. Though in fact the prosecution would be dealing with your attorney, and not you.
For example, you might be charged with burglary, and the deal might be that you plead guilty instead to attempted burglary. That might be an empty deal, if the penalty is the same. Negotiating "residential burglary" down to "criminal trespass" is a reduction from class B felony in Washington to a gross misdemeanor, which would not be pointless.
This deal will be presented to the judge, and you will be interviewed in court by the judge to ascertain that you understand the deal and you freely agree to the deal. Once the deal is accepted and you enter a guilty plea, there is no changing the charges, but that does not mean that you couldn't be charged with some other crime. Your attorney would (should) make the prosecution promise to not file other charges later, and if that promise is in writing, then it would be binding on the prosecution. If you decided to go with an informal gentleman's agreement with the prosecution, then there was no agreement, and you can be prosecuted for other stuff. So always get an attorney.