“I was taught to daven ‘bifnim’ meaning what the siddur says”
To daven bifnim is highly commendable. The drawback is that you are saying what the printer of that particular siddur chose to print. In many siddurim there are a variety of nuschaos in several parts of davening. Common examples include morid hageshem/hagashem, and many other phrazes in davening that some shuls say while others dont especially nusach sefard. One must therefore know what is one’s minhag or mesorah to say or of the shul you are in, and not just read blindly from whatever it happens to say in the siddur you are using at that particular tefillah.