Is it normal having trouble keeping an erection..why??

Ok so i recently lost my virginity and apart from being a newb i think everything is fine-ish??

i was really excited and so was my girlfriend. we were both virgins. we had been having phone sex and just getting really close to each other before we went to the next level and had sex. on the phone i could be really fast if i wanted to or keep it going for a good 15-25 minutes until i would finally finish.

on our first time i was so embarrassed as i only lasted about a minute or two! i felt humiliated and as if something was wrong with me. immediately i apologized as i knew i had let her down. a few hours later we tried again. this time i lasted a little bit longer; about 2-3 minutes this time -__-!

on a separate occasion we decided to have some fun. we were grinding and making out. she was putting her hands and holding on my member; i was as stiff as a tree, i could knock someone out with it! soon after we stopped and i began to relax. we started making out again and i knew something was wrong. i felt really weird on the inside. she attempted to give me oral pleasure. i was in between being kinda limp and stiff. i assumed as soon as she continued; i would be ok. unfortunately not, my member was uncooperative and disappeared.

Humiliated again! i apologized once more. she began to think that im not attracted to her no more; but on the contrary! its her that gets me so riled up! after a few minutes i knew i was ok again, although because of my shame i didn't try and get erect. a few hours later we were on the couch watching tv and holding each other close. we began to make out and i put her hand on my crotch so she could see that it was still working.

ever since we had sex, we hardly do it over the phone as much. sometimes ill do it alone, and she does the same. what i have noticed is that sometimes i start off very erect; and end up NOT finishing for an obvious reason! now i am getting angry and scared to death! when that usually happens, its when im trying to practice and see how long i can go for. so i tell myself that has to be why. and the other reason i noticed is; after i finish one time, and try to keep going again while im still erect. the problem is that right after i finish and try and keep going ill end up losing it after a few minutes.

i did some reading online and i am scared! what if im one of those young guys who fell into the demographic age groups and im going to have to use Viagra or something!i am trying to relax and not stress so much over it. i was stressed the first day we had sex and im assuming that combined with my inexperience is why i finished so early. but i haven't found a good enough logical answer as to why i lost my erection.

i am taking some dietary supplements and i changed the diet on my food intake. i wonder if this has anything to do with it. someone please tell me its just cus im a newb and need to learn to control it? any info would be helpful, thanks.