The Gemara doesn’t quite say your “complete” sentence. Rashi says what was posted above, that through Tefilla it can be overturned. Mazal of the month is not mentioned in the Gemara, but Mazal of the day is mentioned. Shabsai is considered negative by us as well.
To say, Ein Mazal Leyisroel Aval… makes no sense. If anything, it should say, Ein Mazal Leyisroel Ella Dli. Yes, it is mentioned in Sefarim that Mazal Yisroel is Dli. This has nothing to do with the statement in the Gemara that Ein Mazal Leyisroel.
When you learn through the Sugya in Shabbos of Ein Mazal Leyisroel you see that the Gemara proves this from a few stories where, although the Mazal was overcome, the effect of the Mazal was actually quite visibile.