It is most certainly possible to become a sr dev without a mentor and I’m living proof.
As one of the first front-end devs during the beginning of the Internet Boom, there were no mentors let alone books. We were the pioneers. We set our own rules.
Trial-and-error is not really that slow if you pick up fast and keep at it. Back in the days, I was working on my school home page till they kicked me out of the compute lab. If you have passion, you will learn it regardless of mentors.
In the beginning, I was excited to work with more experienced devs and hope they can teach me all the new technologies. My experience with mentors is not a good one, mainly I never had any because most ppl are just too busy or selfish to teach you stuff. They work all day and when they finished, they just want to go home. My experience is somewhat different because I was never labeled “jr”. Again, we were the first ones ever to do it so there were no jr or sr, only ppl that done it before but for only a few months.
In front-end development, its even more crucial to be able to learn it by yourself because the tech changes so often. Almost every year or every other year, a new de facto framework emerges. I learn all the UI frameworks on my own and I do it in a matter of days.
I always recall my first full-time job and there was a consultant there who was paid massive amounts of money since again this was the beginning of the Internet Boom and nobody knew how much a static web site costs so it was not uncommon that a company paid from $20k — millions for a web site. And so the consultant was shocked that I was able to do stuff that he can do only I was getting the starting salary of $40k/year. The lesson here is don’t get bogged down by labels like ‘jr’ or ‘sr’. You’ll be surprised that you sometimes you know more than the sr guy.