Did you learn the letters in hebrew before or while the lessons? I am more concentrating on trying to remember where the letters are on the Keyboard than really learning them... I think that at the Moment i cant remember them by my own...
I've been studying them during the lessons. At first all I recognized was "seen" and final "mem" but I've been writing the letters down and forming simple words until I memorize the letters and how they work. It's pretty easy, because unlike Arabic, the letters don't connect with rules for beginning, middle and end. There are only a few final letter changes, so it's not as big a task as I though it would be. It's exciting to learn because now I can navigate Strong's concordance more easily too!
Try to write a lot of words...your name for example in hebrew. I taught them myself some years ago while i was listening to hebrew music and read the lyrics. At first I needed to look up most of the letters, but its worth the time ;)
I've also learned the "Aleph Beth" before and I used to speak it a bit in the past but the app is being very useful for me
You might want to try another app like Memrise to get the alphabet down. Or even play with some flashcards or something. I knew the alphabet and had taken some university level and casual Hebrew before Duo but I'm making flashcards with what Im learning. I find flashcards work well for me. There's also a Hebrew script of cursive form that threw me hard in the past (and my university teacher who was a native speaker jumped to the script within the first few weeks so I was terribly lost!) So I've found by writing out flashcards in the script I've gotten pretty comfortable with it. That alone may be a reason to do flashcards or get a blank notebook and start writing each new word you learn down in Hebrew (and the definition and even a transliteration to help you pronounce it). It will really give you writing practice and help you feel more confident with the letters. But actually looking at a short course or something might be good just so you get down the names of the letters and their sounds (since as you've no doubt seen some letters make two different sounds and a few, the resh, tsade, het, and kof or ר, צ, ח, כ as well as the tsade and kof sofits that come at the end of words don't have direct English equivalents.)
I can tell you from experience too that it gets much easier to learn if you already have the alphabet down. I struggled for a long time in that class I mentiomed that moved too fast because of that. The good news is there's less letters in the Hebrew alphabet than the English one so it's not as hard as it seems to learn.