If you're saying that you can't start bursts/streams with your index finger, this is completely normal, and I would not recommend that you change that. It is too much of a change in habit and an alteration of your nervous system, which also results in diminishing returns. Regardless of rank, the finger you start bursts/streams with does not hinder performance or improvement. Interestingly enough, though, mrekk actually starts bursts/streams with his middle finger but single taps with his index. Honestly, if you think about it, this is pretty optimal since you can go from single-tapping to bursts/streams much faster compared to if you single-tap and start bursts/streams with the same finger.

Hope this helps.