Male Sex problems, Can anyone explain whats going on?

Im 21 years old. Long story short.. i broke up with my ex of 4 1/2 years.. we used to have sex on a regular basis, no problems at all.. now i have a new girlfriend, shes wonderful, one problem tho.. my penis wont get hard???? we can make out n all that stuff, and it would get fully ahrd for about 30 sec and then gradually go back down, never to get up again no matter how she touches me or anything.. we've only been together for 2 weeks, so obviously we arent active yet, we want to wait.. but this is very odd and frustrating to me.. ive never had this problem before, with my ex i would get 'stiff' just talking to her on the phone lmao.. whats going on? i know that in the beginning it was performance anxiety (ive researched).. but now im very comfortable with her..a nd yet, it still doesnt want to stay up.. this has never happend to me.. and when we get 'frisky', i find myself shivering and shakey.. i have no clue wutsup, im to teh point where im going to try a fourth of a viagra pill and see how that goes lol.. help? tips? remedies? explanations? i know this is somewhat normal, so im coming out and asking for this help from others who might have overcame it. thank you.