It sounds like you're being given work that's outside of your comfort zone and at the same time being given strict deadlines.
I've been in the same situation as a junior dev so I think it's common in the industry and not necessarily startup specific.
The only way around it, I think, is communication. It's classic managing expectations territory
Give pre-warning: When you're given a new task outside of your comfort zone make it clear that you'll try but may have problems because you have no experience with X. You're junior, not an experienced contractor, so this is totally acceptable.
Fail Fast: Half way through the task, give people warning if you are behind schedule and might not finish in time. Fail fast so that your team can adjust and pivot.
Inform Management (maybe): If this keeps happening and the tasks are consistently way out of your comfort zone (too far out to pick things up through on-job training) then consider pulling a member of management aside (project manager, Dev team lead) and explaining to them in a meeting that you think the role you have been assigned requires someone with more experience. It's not personal, it's about the role the project requires not your performance. They may be expecting you to learn things on-the-job so only do this if you are consistently missing deadlines.
The point is that it's ok to fail or be stuck. As long as you make your position clear to the rest of the team then there should be no hard feelings. You have the protection of the "junior" bubble. However, If you wait until deadline-day to tell people that you're struggling with the task then people will hold it against you.
When I was in this position I had to cut-corners to meet deadlines and then spend my personal time going back and correcting/patching things that I had done wrong, learning in the process. Eventually, I didn't have to spend my personal time programming because I had learnt enough to do it right the first time.
This stuff is HARD. I wasn't able to build the confidence to follow the above advice until 2 years in industry just because I was nervous, I didn't want to admit that I couldn't do something. So don't beat yourself up if you don't master it immediately.