DO I have PMDD or just really bad PMS?

PMDD is not widely recognised as a "real" disorder, and a lot of people see it as a pathologisation of women which pharmaceutical companies can use to sell more drugs. It's most likely that you just get bad PMS. Different women manage their symptoms differently, you could start by looking on google to see if there are any methods that seem like they would work for you, and if you're really worried, or nothing you try works, see your GP.