Mom meltdown

My toddler is 3 years . Since around 5 months he started having a realy bad tantrums and he won't listen or do what i say unless there is a big tantrum beforehand. I have started potty training in august and it went great but now he won't even step foot in the bathroom without fighting. I have tried every method there is but he'sjust stubborn. I'm asking for an advice on how to deal with him because i just went into my room and had a meltdown from all the stress. :(