I have learned from my mistakes with this as well! The Portuguese verb 'pensar' takes the preposition 'em' (I don't know why - it just does) if the meaning is 'to think about' or 'to think of' someone or something. It can happen that the 'em' then disappears as it has been contracted with a following 'este', 'esta', or 'isto' etc to give 'neste', 'nesta', 'nisto' etc.
"Pensar em" is correct in this sentence, but "pensar" is followed by "que" when it acts as a transitive verb.
Ex: Pensei que ela fosse competente, mas ela não é.
In order for pensar to be a transitive verb it must have a direct object, which in your example it doesn't. Secondly, the indicative instead of the subjunctive generally follows pensar que in affirmative sentences. Your sentence should be: "Pensei que ela era...."
"Pensei que ela fosse competente, mas ela não é." The words in bold represent the noun clause which functions as the direct object of "pensar". There are many similar sentences on DL. Perhaps you've seen these:
• Eu pensei que ele me entendesse.
• Eu pensei que você fosse médico.
This is an explanation from ciberdúvidas re: "pensar" + the direct object clause:
"Na frase apresentada (norma brasileira) 'eu penso que você é útil para mim.' A frase que se segue a "eu penso," introduzido por "que," é o objeto directo da oração anterior." https://ciberduvidas.iscte-iul.pt/consultorio/perguntas/regencias-de-crer-e-pensar/21131
Secondly, the subjunctive follows "pensar" in affirmative sentences when what you were thinking turns out to be a mistaken belief. It's counterfactual which is why the subjunctive is triggered.
Eu pensei que você fosse médico, [mas você é apenas enfermeiro].
I wrote exactly that and it marked it as wrong and came back with I'd never thought about that, which is still right in the English language but my version is correct as well