I have maintained my 150 days streak of learning English on a mobile app that teaches Englishn app.

Amazing! Nothing too wrong here!

Something that I've done recently that I'm proud of.

Something that I've done recently that I'm proud of.

I've managed to maintain a 150-day streak of studying English using a language-learning app.

I have maintained mya 150 -days streak of learning English on a mobile app that teaches English.

I have maintained my 150 days streak of learning English on a mobile app that teaches English.

In terms of a streak on an app, native English speakers would tend to say '150 day streak' instead of '150 days streak', despite the fact that the 150 is a plural quantity of days.