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The Milram team rider's decision follows the provisional ban he received on Wednesday from cycling's ruling body the UCI triggered by his positive test for EPO in an out-of-competition control in August.
Sentjens, who had originally denied any wrongdoing, held his hands up to acting illegally in a statement on his personal website.
"I didn't ask for the second 'B' sample (to be tested) because I knew what it would contain," he wrote.
"I want to apologise to everyone who believed in me." The 29-year-old cyclist went on to explain how he had come to obtain the drug.
"I wanted a contract, I have a new house, a son, a car...I made a mistake. In a flash I drove to Barcelona. I was set to visit all the chemists where I could find EPO. From the second (pharmacy) I had what I wanted.
"On August 16 I was tested. I knew already that I was finished." EPO (erythropoietin) can significantly enhance performance by boosting the oxygen-rich red blood cells in the blood.
Sentjens, who was born in Holland but took out Belgian citizenship in 2005, retires with four wins after nine years as a professional rider during which time he rode for Rabobank (2002-2006), Predictor (2007), Silence (2008), then Milram.
Sentjens' frank admission follows Wednesday's statement issued by the International Cycling Union.
It read: "The decision to provisionally suspend Mr Sentjens was made in response to a report from the WADA-accredited laboratory in Cologne indicating an Adverse Analytical Finding of Recombinant EPO in a urine sample collected from him at an out-of-competition test on 16 August 2010."
