When I was on Prozac I had decreased sensitivity. I switched to another medication and the issue went away. Masturbating twice a day may be excessive and you may be decreasing your sensitivity because of that. Also the brain is the biggest sexual organ and watching pornography may dull your sense of excitement when with your gf. Also I would try and use some lotions to help soften the skin. Like palmer cocoa butter lotion. Dry skin on the penis can also decreases sensitivity. Also if you are applying too much pressure when by yourself, you may be desensitizing your nerve down there, and there is no way a vagina can match that.