What should I do

If I had a dime, for every time a client told me their criminal conduct was an accident :)
OK, whether it was or not, Walmart prosecutes shoplifting. You may well get a Motion to Revoke your other community service and have to deal with the new charges as well. You need a criminal attorney ASAP to see if they might be able to deal with Walmart not to pursue it some way. I am just saying that is doubtful in my mind, but don't get what you don't ask for.

Retain a lawyer and they can help you keep it off your record, Deferred is an option.

I'm very old and forgetful - you can ask my wife - and I have never "forgotten" to pay for things at Walmart. In fact, I have left things that I had already paid for.
You are among friends - it is OK to admit that you didn't think you'd get caught. It's OK to say that you didn't think it would matter. But, when you tell a lawyer that you did not intend to steal something, you go to trial and when you go to trial, there is only win big/lose big. MOST OF THE TIME, it is LOSE big.
Hire a lawyer. Hire him now. MAYBE he can keep you from getting a theft conviction AND keep you from getting thrown in jail on the marijuana case PLUS losing your license for 6 months.