What Causes Erectile Dysfunction?

Question: Hi, I am 35 years old and got married 6 months back. After marriage, when I started having sexual contact with my wife, the first few weeks were not successful, as my Penis lost erection after insertion or during insertion process (whenever i had to push to insert). After that, my success rate was 50-50% and I started developing a mental block, as I am not sure, when it would be successful and when not. Recently, 2 weeks back, I consulted my family doctor for a regular checkup, he said my cholestrol values were too high (328mgs, LDL - 205 mgs, hdl - 73 mgs, triglycerides-350 mg) and gave me Rosufast tablet (1 tablet every day). I have been taking this tablet for past 2 weeks, and now for past 4 days (first time after taking tablet), whenever i try to have sexual contact with my wife, i face the same erection problem. It always seems to be 90% hard, and when i try to insert it softens up. I am worried whether this is due to the tablet i am taking or whether i have ED problem. This is hurting my confidence and am worried that it would start affecting my marriage relationship. Please clarify the next steps i need to follow and are there any tests that I could do to find out whehter i have an ED problem or not.