Not only could that mere 10 minute lunch be breaking labor laws depending on where you live, but the implication that you don't have value a couple months in is a huge red flag. They hired you, therefore you have value to them, and they are invested in your success. Either they're trying to play mental games early on to squeeze more out of you than is reasonable, or they're not doing their job in providing enough resources to get the job done under healthy conditions. Itās worth noting whether this is a company-wide culture issue, or if itās just your direct manager too.
I don't know the job or what the boss meant by 'work harder,' but it can take anywhere from 1 to 2 years to truly learn a job. Given that it's also your first 'real' gig, there's a whole other dynamic of getting used to a workplace environment, and it's their job to help acclimate you to it. Keep your options open, "manage up" (give honest feedback to your manager about how you feel), connect with your colleagues in the meantime to put pressure on management to change their ways, have your resume updated. If you're starting to burn out and you need to jump ship, it's best to have things in order already, so that stressful situation isn't made worse by feeling like you have to do even more stressful tasks in order to find something else.
tl;dr thatās ass.