This is a really good thing to learn from, although I doubt it felt like it at the time. The key thing that every junior dev needs to take from it is that it's fine if things don't take however long the estimate is so long as you're regularly communicating with the PM. PMs hate surprises. They need to know exactly what the state of the project is at any given moment. For them to think that a story is progressing on track when it's not is very, very bad (in their minds; it doesn't actually matter most of the time.)

If you work on a story that's estimated to take 3 days and come back after 3 days saying "It's not done yet. I'm only half way through. I had to learn a ton of stuff!" then that knocks out loads of other things, and people get pissed off.

If you work on a story that's estimated to take 3 days, and in the stand up on day 2 you say "I think I'm going quite slowly because I'm having to learn a lot, and I don't think I'll deliver this in 3 days" then your PM will (...should?) respect that you're keeping them informed. The senior might even step up to help you get there faster.

Practically all the actual work a PM does is communication. If you're not communicating with them then you're blocking them. No one likes that.