maîtrisais

/mɛ.tʁi.zɛ/maîtriser
1.
verbInd1stImperfectSing1, 2

was good at

had mastered

knew how to do

À l'époque, je maîtrisais bien l'espagnol.

Back then, I was really good at Spanish.

2.
verbInd1stImperfectSing1, 2

had (something) under control

was controlling / was restraining

Tu maîtrisais la situation avant qu'il n'arrive.

You had the situation under control before he arrived.