Want out of the Marines but do not want future to be completely unbearable.
You can only receive a Dishonorable Discharge at a General Court Martial. If you are charged with one specification of drug use and nothing else, you are more likely going to find yourself at a Special Court Martial, Summary Court Martial, or an NJP. Your command will decide. If you are convicted at a SPCM, you face the possibility of a Bad Conduct Discharge (BCD), up to one year of confinement in the brig, forfeiture of pay for up to one year, loss of rank to E-1, and a federal conviction on your record for the rest of your life. If you are not discharged criminally at a SPCM, you will face administrative processing with the high possibility of receiving an Other than Honorable discharge (OTH). Consult with a Judge Advocate for a complete picture.
You can expect to be negatively highlighted and placed on your command's legal tracker as soon as your urinalysis is returned positive. You can expect to wait in this status until your punishment and processing. You can expect the possibility of brig time if you are convicted at a SPCM or SCM. You can expect to be asked for the rest of your life on school and job applications about the characterization of your discharge.
Only you can answer whether all of that sounds unbearable. I think that it would be much easier to do your job and fulfill your commitment. Your future self will thank you.
Your inquiry does not disclose the status of your case, or if their is a case at all. Nevertheless, for the sake of this answer, I will presume your command has evidence, either through urinalysis results or witness testimony, concerning your alleged marijuana use. I will also presume you are enlisted and not an officer. Marijuana use is a violation of Article 112a of the UCMJ. Assuming the only charge is use or possession of less than 30 grams of marijuana, the maximum punishment If convicted at a general court-martial (GCM) is a dishonorable discharge (DD), 2 years confinement, forfeiture of all pay an allowances and reduction to the lowest enlisted grade.
However, if the only allegation is marijuana use, it is highly unlikely your case will be referred to a GCM. Rather, your command will dispose of your case through either a special court-martial, summary court-martial, or non-judicial punishment (NJP). None of these dispositions authorize a dishonorable discharge. That said, a special court may impose a bad conduct discharge (BCD) and a summary court or NJP might be followed-up with involuntary administrative discharge. Involuntary discharge could result in a service characterization of under other than honorable circumstances (OTH) or under honorable circumstances (General). You'll be entitled to a hearing before a board of officers if your command recommends an OTH discharge or you have more than six years service.
Negative service characterizations carry with them significant collateral consequences. For instance, your eligibility for VA benefits, including GI Bill and home loan guarantees, can be restricted and in many cases barred. In addition, you might be disqualified from Federal, state and local government employment. Your ability to obtain student aid, scholarships, and admission to higher education institutions might be impacted. It could effect your ability to obtain licenses and certifications needed for professional employment, as well as your ability to obtain credit and loans in connection with establishing a business. Civilian employers may consider your military service characterization when deciding whether or not to hire you. Even if hired, a negative service characterization could impact your opportunities for advancement.
Bottom-line, a negative service characterization, whether DD, BCD, or OTH, will have a significant impact on your life. You should retain legal counsel, either through the Marines/Navy or from the private bar, to discuss in detail your particular circumstances before making any decisions concerning disposition of your case.