You sound exactly like me. Misdiagnosed for years and expected to hold up a normal life while this underlying issue goes unsolved. People do think I'm mean because the overwhelming surroundings makes me angry. Usually in a child, they're scared, but I think in undiagnosed adults it turns into anger after years of having to deal with it. I'm known to be normal one moment and then something happens that no one else notices and I lash out at the person I'm with. Of course, being misdiagnosed, it was just seen as anger problems due to my borderline personality disorder (which I do actually have). I'm having trouble finding a doctor that will understand, so if you're google savvy go that route to find a good doctor, or use the occupational therapist state directory (google "find occupational therapy".
As for coping skills I've developed without doctors help, headphones and ear plugs. I can't handle noises, so it helps for that obviously. But for the meltdowns, go to a quiet dark room and put in headphones with your favorite calming music. Listen to the album, and the meltdown can be (if you're like me) averted or at least minimized. I can't help with social situations as they still give me meltdowns, but I've just grown to accept I can only handle small groups of people who aren't intrusive sensation-wise (quiet people with calm voices, people I've known and trusted for many years, and always on my own "turf" aka my house). People who have actually gotten help for this disorder can help you way more. Things can and will get better, simply knowing of SPD is a huge step toward managing SPD life. Read these boards, they're filled with coping tips and such. You're not crazy, you're not alone, and you're not unfixable. You're just untreated. Now that you've discovered SPD, talk to whatever doctor you have that can help you find the right doctor. I wish you absolutely the very best of luck, I really hope you find the help you need.