When a person is charged with a criminal offense, broadly he only has two choice. The first is to fight the case, all the way to trial and put the state to its burden of proof beyond a reasonable doubt. The second is to accept a plea offer and give up rights to challenge the evidence and confront the witnesses against you.
In my experience, criminal defendants who take pleas -- even favorable ones -- frequently wonder what they cost themselves by not taking the case all the way to trial. Here there is some damaging evidence against you. If the residue of methadone is enough so that it can test positive in the state's labs, you can be charged with possession of that substance. The presence of an unusual amount of cash in conjunction with drugs raises an inference that a defendant is involved in drug selling activity.
True, the state would have to prove the charges against you beyond a reasonable doubt, but in order for it to come out that the money was your rent money and your tax returns and not the unlawful proceeds of drug sales, you would have to take the stand and corroborate that. And once you take the stand, at least some aspects of your prior conviction would be admissible against you.
I imagine that your lawyer went over this with you. That's his professional obligation: to give you the strengths and weaknesses of the case you are fighting, to convey and discuss the up and down side of every plea offer made to you by the prosecution, and to put you in a position to make an informed and intelligent choice of how you wish to proceed. Once the lawyer does that, the choice is up to the client, and even if the client chooses to reject a plea his lawyer recommends, the lawyer will try the case and is required to mount the best defense of which he is capable.
A required choice between two equally unattractive alternatives is the toughest decision a person ever gets to make, and that's never more apparent than in the criminal justice system. You chose to cut your losses rather than risk a conviction and a potentially heavy jail sentence.
It is almost impossible to get a plea back after you've been sentenced, because judges generally see to it that a record is made at the time of the taking of the plea that is designed to make that plea airtight. However, you can talk to a post conviction lawyer about whether you have a basis to take your plea back and go to trial if that's what you really want. If you do get your plea back, however, you would start from square one. That is, any reduction in charges for the purposes of a disposition would be taken back with the plea, and you'd be fighting the original charges and facing exactly the same dilemma you had in the first place. If you're not successful, you won't do better than you've already done, and you could do a great deal worse. So think carefully.
You didn't actually ask a question, so if I didn't touch upon what you wanted to know, please reply and clarify.