I assume you mean you want to alternate strictly M-I-M-I...
Does what you are doing now sound bad? is it causing you problems?
Raking often sounds better than alternating anyway - and in any case, the most important thing is the rhythmic feel of the song, which might sound better if you don't strictly alternate.
So you might start out playing a song with the easiest fingering, and then start working out finger patterns that sound better - and at no point find that you're doing strict alternating. That's perfectly reasonable.
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