First of all, NO religion has ever existed in a "vacuum". Just like people of today, individual and group beliefs are effected by contact with other religions and philosophies. Personal and cultural interpretations of religion change over time, and go through relaxed and conservative periods
Your anxiety is probably due to clinging to your religion as an unchanging "crutch" rather than believing what you want to believe and keeping that separate from history. I am not saying to bury your head in the sand and ignore the history of Christianity (like many "Fundies" do), I am just saying that if your religion works for you, then it does not matter that it has gone through changes and been influenced by other beliefs.
Early Islam was heavily influenced by (Roman Catholicism) Christianity, Judaism, and Zoroastrianism.
Early Judaism was heavily influenced by Zoroastrianism, and the polytheistic beliefs of Babylon, Sumeria, and Egypt.
Egyptian beliefs were heavily influenced by the polytheistic beliefs of the Sub-Saharan African religions, the Greeks, and the Zoroastrians.
The Romans were heavily influenced by the Greeks, Zoroastrians, (Iranian) Babylonians, (Turkish) Phrygians, and Hittites.
The Greeks were heavily influenced by the all the same as the Romans plus the Hindus, Buddhists, and even the Chinese.
And just like everybody else, early Christianity was heavily influenced by the Romans, Greeks, Egyptians, Judaism, Buddhism, and everyone they had been influenced by.