Thanks for all the comments and answers! Also, thanks for pointing out the other question – I've read all the answers to it and took away (hopefully) as much as is relevant to my situation. I think, however, that my case is different. I don't have trouble with figuring out what to do, it's just that nobody with more experience is checking my outputs. So I know the code works and that I've done my best, but I can't catch what I don't know. However, judging from the answers and comments to both mine and the other person's question, this is probably quite normal, so I guess I'll just have to live with it.
Also, to clarify: formally, the coworker is not my supervisor, we both fall under the same manager, who is responsible for the whole department. Informally, it is very unclear. My coworker is understood to be the senior member of the team, and she was introduced to me as the team lead of our two-person unit. HOWEVER, at my three-month review, I mentioned that I felt like I was doing a lot of administrative stuff that, in my opinion, should be the team leader's responsibility, and was told by my manager in no unclear terms that the "team lead" role is very informal and that it is up to the team to decide who does what.
To sum up, I think that what I really needed to hear was Rose's comment and especially JBH's answer – THANK YOU! I'm just going to accept the fact that I've been put in a different position than I expected when I was hired, and try to really own the app and carry out all the decisions the other person leaves to me the best I can (I'm also going to stop referring to her – in my mind – as my senior, because it just frustrates me). I love working on our project, but in the end, I'm not designing the next Apollo – if it crashes once in a while, no one dies and the company won't go bankrupt.
I currently work as a frontend developer, but my background is in an unrelated field (coding used to be a hobby). When I decided to make a career change, I taught myself as much as I could, made it through a coding bootcamp and finally landed a junior dev job – this is where I work now. I’ve been here for 10 months.
There are two of us working on the frontend part of the project. I was hired as the junior and the management sees the other developer as my senior, but the trouble is that she's not really acting as one. Her main expertise lies in CSS and design and unfortunately, after 10 months on the job, I know both the framework and our codebase much better than her. I feel she always picks the easier tasks/stories/bugs, and I literally have to force her to do code reviews for me and she never tells me anything except "it's okay". I am constantly terrified that something will go wrong, because the app is being developed by an unsupervised junior. It's me muddling through by trial and error with the help of StackOverflow.
On the one hand, I see it as a great experience – I am basically doing the work of a mid-senior developer and have zero official responsibility. On the other hand, my code is probably really bad, and I would really like to be mentored and taught and supervised. Also, my work is not visible - everything we produce is seen as the work of the frontend team, so I am not "proving myself" capable of handling larger tasks or anything. Also, I don't feel comfortable taking on any advanced responsibilities, because I feel my experience is really very junior.
My senior has been with the company for a long time and is respected and well-liked (she's a very friendly, extroverted person). I am in no position to even hint that she's not doing a good job. At the same time, I have become very resentful of her.
What can I do? I can probably switch projects, but it will take at least another six months. The company is great, I don't want to leave. Is there a way to get more formal training while staying on this project without implying that my current senior is not doing their work?