I can’t nut

My girlfriend (F18) gave me head (M19) for around 2 hours in 30-30 min segments and I couldn’t nut whatsoever. We just do oral and no penetration due to some personal reasons , anyway no matter what she does ( boob job , ass job , feet job , head or even a classic hand job) I don’t orgasm. And she feels like she isn’t trying hard enough and shit even though I have alot of pleasure while it’s going on . I really enjoy everything she does to me and she works hella hard to pleasure me sexually but if it’s not my own hand I cannot nut. I hate to see her in self doubt and feeling bad because of me and this is causing me to fall into the previous self hatred problems I had , I have tried to reassure her that she is doing great and it’s not her fault and I love her and she knows that but subconsciously she feels shitty and it’s unacceptable for me all because of the reason I cannot fucking cum unless it’s from my own hand . I’m not sure what the problem is . Is it mainly because we haven’t even done penetration and that it is required for guys to cum? Is it because her technique is wrong? Is it because of some other unforseen issue which I’m not sure about?

Note : she is my first and only girlfriend ever. I haven’t actually ever had sex , and both of us still have our v cards . I don’t really have alot of experience so ummm yea…. That’s pretty much it .

Edit : this has happened several times . First 2 times it was just simple handjobs (she was still learning didn’t know shit about how to stoke my dick ) , then for the past 6-7 times it was 50/50 handjobs and blowjobs with a little bit of ball sucking as well and the last time it was a hella leghnty blow job .