Welcome to the world of majority aspirants. For most, English is not the first language and it is common to hear people say that they think in their mother tongue before speaking in English.
Coming to the SSB, this is a test of your personality. Clear communication in Hinglish is way better than mumbling in English.
You also need to understand that as an officer, you will command troops in Hindi most of the times. English is majorly used for official communication and during officers' interaction. I am sure, that much language skills are easy to get.
In the interview, the focus is on assessing your OLQ or Officer Like Qualities (OLQs) and if English were to define that, then every aspirant fluent in English should be the only one getting through.
That is not the case for sure. Develop confidence by understanding what is needed for each stage of the interview. Improve your overall personality by focusing on your core weaknesses and put language last in the order of priority.
If you are still in doubt, then think about successful military leaders such as Napoleon, who never spoke English and yet led conquests well!
English never defined military leaders but their personality did.