I think most people at most jobs with stressful deadlines, engineering work, and where individual productivity is measured/praised, feel the same way you do. You aren't crazy or broken for feeling the way you do. Most companies/engineering departments talk about trying to nurture a healthy/pleasant working environment, but that doesn't mean most companies succeed in those efforts. With clients and bosses demanding things constantly, it's not surprising that work environments tend to fall into a negative state despite the noble efforts of a few employees or a VP that talks about building a pleasant work environment.
It sounds like you didn't love your last job, you tried a new type of company (in terms of size of employee count), and you don't love this job or long term project you are working on. There are pleasant companies with fun projects out there to work for, but they are much less common than the companies that suck to work for. Maybe you should write out a list of attributes that you assume an ideal employer/project has, and then start interviewing for those companies/jobs. If you hate your next job as well, rinse and repeat.
Remember your manager is your boss not your friend, so opening up to them that you are unhappy could lead to you being let go or whatever. If you aren't financially stable for 6 months, I wouldn't even tell them this stuff, I would just start prepping for job interviews during off-work hours and try get a new job. If you are comfortable not earning a paycheck for 3-6 months, I would tell my manager this stuff, see what they say, I would not go to HR before manager unless my manager like physically/sexually assaulted me or something. If manager wasnt helpful, I would then go to HR.
At my last job I felt like i was pegged as the dummy on the team, no big deal lol. I think i deserved to be pegged the dummy, maybe you do maybe you don't, regardless of that, you shouldn't worry about it or burn yourself out trying to impress your coworkers because of it (this could all be in your head as well).
Just know, if they are going to fire someone, it might be you and not one of the other all-stars on the team. Try accept that this is out of your control and not stress about it.
What did I do in my last job that i was unhappy with? I spent a few months prepping to interview in off-work hours, eventually I asked for a raise, got denied my raise, interviewed around for a month, and got a new job.
FYI
I've worked at a Fortune 500 company (big IT consultant 100,000s of employees), two midsized company (1000ish employees), and two smallish companies > 300 && < 1000. Most places sucked day to day TBH.