This has turned out to be a huge wall of text, sorry.

Most autistic communities recognise that an official diagnosis is hard to get and not always helpful/necessary, so while it's wise not to say as much out in the rest of the world, it's completely valid to self-identify within an autistic space.

Feeling like you're faking the whole thing is so normal. I didn't have a self-identification journey before being diagnosed as an adult, and for the first two years after that diagnosis still felt like I wasn't actually autistic and the assessors made a mistake, or not autistic enough to "count" and give myself accommodation. It comes from a lifetime of learning that your experiences are "wrong", that you "can't trust" your own interpretations of the world around you, that you could do anything a neurotypical person can do if you just try. We late-identified are raised to doubt ourselves. It's often no one's fault, but it still takes a lot of unlearning.

What you do now is start regularly checking in with your senses. Are you feeling tense because you've been ignoring a too-bright light or an irritating noise or an uncomfortable piece of clothing? Removing those kinds of subconscious distractions is a huge relief, whether that's with headphones, earplugs, sunglasses, baseball hats, whatever works. Are you hungry/thirsty/sitting in a really uncomfortable way and haven't realised it? Solve those needs.

Then look into stimming. Physical or mental actions that you feel an urge to do and find calming. They don't have to be "weird" or socially unacceptable - spinning a pen, fiddling with a paperclip, chewing gum, twisting bracelets/wristbands/rings, squeezing a stress ball, wearing clothing that puts pressure in certain places (e.g. I wear dresses with a waist belt for pressure on my abdomen) are all possible stims. Even if it sounds completely unnecessary, try a bunch of thing and see what makes you feel safe and calm.

Chances are that if you're autistic you'll have a lot more mental energy after discovering your sensory sensitivities and stims, because it's not all going into trying to ignore all the discomforts.

The social side is less straightforward, and to be honest I've not figured it out yet. People say masking is damaging and should be avoided at all costs, but those costs can be significant. I've tried to go the route of not masking stims (having found subtle ones that work) and unapologetically wearing sunglasses indoors, but still put on the mask when it comes to communication, tone of voice, facial expressions and the like. Because my aim is to be understood, not to take a stand for all autism-kind, and the relief of meeting my sensory needs frees up a lot of the mental energy for it. Maybe that's the wrong approach, but it's the one that poses the least risk to my comfort.