Hello, Thank you for using HCM for your questions! First of all, your condition is rather common among males and it is becoming increasingly important among young men as well (due to diet change, pollution, environment, life stress, etc.). Secondly, the situation you are experiencing is a very frustrating one and I think that there is no male that did not experience it and the embarrassment associated with it at least once in their life. Therefore, from this point of view, you are not alone! Thirdly, you should not make a tragedy out of it. It could happen to anyone and if anyone says that it has never happened to them then they simply lie! Now, what could be the causes of your situation? There could be infinite reasons, but the most common include: being too stressed and being too eager to engage in sexual relationship (you like too much the partner). Your body parameters are all OK (although there is no correlation between body measures and erectile function), and I assume you are healthy. Also, the size of the penis is quite good and the change in its size in two different measurements could be attributed to different levels of erection in each case. Therefore, in the absence of other health conditions (problems with thyroid gland, etc.) i think that your problem is mostly mental. Please, do not underestimate the role of psychology or brain in healthy sexual life. There are many cases of men who get hard rock erection during the night or when they are not thinking of their sexual partner but, when they are ready to engage in real sexual activity with their partner, they do not get erected and they fail, feeling very anxious about the situation and being terrified about the next sexual encounter whether will they fail or not. And this turns into a vicious cycle (your fear boosts the insecurities of the next sexual encounter): you can not avoid thinking of your problem and the more you think about it the worse it becomes. Therefore, in order to break this vicious cycle you should consider changing some things in your life. Before mentioning these easy steps, I strongly recommend to see an andrology doctor (doctor dealing with male sexual and reproductive issues) because this is the most important measure you can take to overcome your mental block prior to sex. Meanwhile, you can do some other things in order to improve your condition (these can be found also in textbooks, but I am giving you a general summary): eat healthy and engage in physical activity. Vitamin D intake has remarkable positive effects on sexual life of males and you should consider taking it, but talk to your doctor about the quantity. Do not avoid completely foods containing cholesterol as cholesterol is needed to produce testosterone. Therefore, consider swallowing 1-2 eggs each day (only the red part of the egg, not the white. The red/yellow part of the egg contains that you need). In summary, my best advice is: see an andrology doctor and talk openly with him. I am sure that after his advice you will overcome your situation and will enjoy sex with your partner. I wish you all the best!