Un mauvais rêve/Un cauchemar

Both are equally common, but it's always useful to know "cauchemar", which means "nightmare"

J'étais effrayée, alors je ne pouvais pas dormir encore'ai pas pu me rendormir.

Main action of the narrative, focus on the consequences of the actions = passé composé => j'ai pu
s'endormir = to fall asleep
se rendormir = to fall asleep once again (after being woken up)

J’ai voulu envoyer un message aà mon petit ami, mais je savais qu’il était en dormanit.

a = has (form of "avoir")
à = at, on, to (preposition)

"be +ing" continuous forms exist in English, but not in French
The imperfect is enough to convey the idea of a continuous action => dormait

J’ai écouté un podcast et éventuelfinalement, je me suis encore r[restée éveillé.e ?]

Big false friend. eventually = finalement
"éventuellement" means "possibly"

If you didn't manage to sleep between being first woken up by the bad dream and the podcast, then I assume you "stayed awake", which is "rester éveillé(e)" in French
"je me suis encore réveillée" means "I woke up once again", which implies you managed to fall asleep once again at some point