Im 18 And Have Little To No Sex Drive?

Im an 18 year old male, i drink and smoke socially, no more than 4 or 5 cigarettes a week, and drink every now and then (not every weekend) i weight 64kg and am around 185-187 cm tall. At the start of the year i was a crazy sex obsessed teen. However recently ive noticed something isnt right. im not getting the urges i used to ie to masterbate or watch porn i dont even check women out as often as i used too. And the only thing that has changed in my life is ive been working which has caused abit of stress but nothing to dramatic. Ive never had sex however ive been experimenting with this girl im seeing and i just find it downright difficult to maintain an erection. Recently i was getting oral from her and i just went flacid. I can get erections i just have difficulty maintaining them. I am uncircumcised and still wake up each morning with an erection so i guess thats good, and i get another one or two throughout the day, so im guessing it might be because of mental issues. Could stress cause me to lose my libido? i do get nervous when engaging in sexual activities but i always have and i guess its to be expected. So could you guys guess as to what it is i do experience abit more stress than earlier in life so that might be it and i think i should go and get my testosterone levels checked. im also considering trying horny goat weed to see if that helps.and if i do have low testosterone how do i boost it? Im just really worried i dont want this problem at this age....