I have been having amazing results with the flour blend in this book: https://www.amazon.com/gp/product/1615190503
It's kind of expensive and a pain to start up with, but once you get all the different flours it's not so expensive to just buy the ones that run out. The blend has a different amount of each kind of flour, so they don't all run out at the same time. I think it's 6 different kinds of GF flour blended.
The sugar cookie recipe in this cookbook is fantastic. If you don't already have this book I'd definitely recommend it, it's my favourite.
It's not a fun diagnosis, that's for sure, but it's not as bad as you're fearing. There are so many great gluten-free products out there now that you can still enjoy bread, pasta, cookies, cake, beer...for just about every wheat-based food out there, there's a good gluten-free substitute (pasta made in Italy tends to be the best, Udi and Kinnikinnick make great bread, bagels, pizza crust and cookies). It's best if you like to cook or are willing to learn, since you can make pretty much anything you want for yourself at home. I was given this book soon after I was diagnosed, and their recipes are fantastic (not shilling for them, I promise! Their flour mix is just the best). I make my own ice cream, do most of my own baking and canning, and I just got a little deep fryer to play with. It definitely takes some experimentation, but you'll be able to make all your favourite foods in gluten-free form.
As for the ice cream, mayonnaise and vinegar thing, that's not true. You do have to be sure you're buying gluten-free ice cream (it's often cross-contaminated, but there are many safe brands out there, such as many flavours of Breyers), but as far as I know the only kind of vinegar you can't have is malt vinegar since it contains barley malt. I've never seen a mayonnaise that wasn't safe. You'll spend a lot of time reading labels, especially in the beginning, but you'll probably be surprised at how many things you can actually eat. You just have to be wary of hidden gluten, it's in things you'd never expect like soy sauce, cornflakes, sausages...all sorts of things. But if you carefully read labels, you should be fine.
For eating out, yes, it can be difficult. Fast food is pretty much out, as are the majority of restaurants, but there are increasing numbers of places that are willing and able to serve celiacs. My protocol is to first look for places that advertise having gluten-free options, then look around for reviews that mention said GF options (bonus points if the person doing the reviewing has celiac) and if the reviews look positive, I call the restaurant and talk to someone there. You can usually tell in seconds if the place is safe, just by the way they respond. I've had a few people confidently say something like, "Yes, we offer lots of vegan options!". Those are places I don't go to. But if they start talking about cross-contamination-avoidance procedures, dedicated fryers/ovens/prep areas, and the types of gluten-free food they offer, then that's a good sign. If I decide to go to a place, I always tell the person seating me and the server immediately that I have celiac. Then I ask a lot of probably annoying questions about the food, and then if all goes well I leave a big tip as thanks for putting up with me. I've only been glutened once while eating out, so it's absolutely possible! It's harder when socializing with friends at their houses, or at events like weddings. At those, I either bring my own food or just eat beforehand. It's a bummer, but it's better than getting sick. It's also hard when traveling. I always loved trying the local foods when I was abroad, and that's pretty much out now. I haven't been anywhere where I don't speak the language since I was diagnosed, but I know that people manage.
I'm also lucky in that my family has been very supportive. My husband willingly avoids gluten-containing foods around me, and eats entirely gluten-free with me in our home to keep me safe. When I visit my parents, my mom cleans the kitchen like mad and sequesters all gluten food in a plastic bin for the duration of my visit (probably overkill, but nice). The first Passover after I was diagnosed, my mother-in-law surprised me by preparing or buying gluten-free versions of all the traditional foods I thought I'd be missing (matzah ball soup, gefilte fish, noodle kugel, she even made brisket with a gluten-free version of her usual sauce so I could eat it).
Finally, keep in mind that if it is celiac, you'll feel SO much better going gluten-free that it will be worth it. When I was diagnosed, I'd been sick for almost 5 years, and I didn't remember anymore what it felt like to actually feel well. After a month of gluten-free, I felt like a damn superhero, I couldn't believe that most people feel so good all the time! That feeling definitely helped me accept that this was the hand I'd been dealt so I could move forward and make the best of it.
Good luck!