I keep getting tasks that are above my skill level. How can I address this without coming accross as grossly incompetent?

I’m 6 months into a new job that uses a technology I’ve had no experience in (but am trying really hard to learn). Often I get tasks that I have real difficulty with. I flag this: I say that I’ve never done that before and I’ll have difficulty. I get some help, but the task always takes way longer than estimated and is done pretty badly. This puts me under a huge amount of stress and is embarrassing as my tasks often get rejected at QA.

I’m all for learning but its just too much. I need to master things incrementally, not all at once. Or else do a proper course on the subject.

How can I talk to my boss about this without looking like I’m useless or backing away from challenges?