I used to really struggle with this. In time it will probably go away, it did for me. I'm 24 now and struggled with it until I was 23. I did a lot of research and these are the things that stuck out to me, even though nothing fixed it, the following things at least helped me know what was going on.
A main thing was needing instant gratification. Not just with sex but with many things. I struggled with school and keeping up hobbies. Even reading and trying to learn things like piano or cooking. Browsing reddit and other things I would read the title and the first comment only. I would never actually read the article or even a comment that was too long. This problem segued into the other problem which is bad habits with porn.
I don't know how legitimate it is because I've done lots of research and there is not a clear consensus one way or the other. However in my gut I believed it is true. Muscle memory is a real thing, including the muscles down there and how your mind interacts with them. When I watched porn I would try to make myself last a long time, but when it came to that point, there was not much I could do, even if I backed off for 5 minutes when I started back up again I would be right there ready to come. Another problem was when I was ready to come my determination to try to last longer would go out the window and I would just finish because it felt so good and I wanted instant gratification.
Further more most times I watched porn I skipped through a few parts of the video until I found a part I liked and would finish. If I wanted to I could get on the computer, start watching, and finish all within a few minutes. For me this was BAD practice because I feel like it was teaching my mind that when there were tits, it's time to come. Sorry if I'm getting to into it and this might be not similar at all to what you are going through but maybe some people out there can relate to this.
Unfortunately I never found a remedy. Over time the premature ejaculation just stopped. I did go through a phase where I tried masturbating in the bathroom before sex. I also tried drinking alcohol which DID help, but only slightly. Xanax is something that really helped. I also had trouble with pain killers and eventually harder types of opiates. These two drugs specifically made me last for hours, however it was not ideal because my erections would only be like 3/4ths hard and the girl I was sleeping with would get upset that I couldn't come and blamed herself. She knew I was on drugs and that it was the drugs that were affecting me but she still was self concious.
Once I got off the drugs the balance was finally there. I could last a long time and control it MUCH better. I honestly think that this just came with time and age. You could MAYBE try alcohol like above comments suggested and MAYBE xanax (not painkilers, seriously) but only small amounts and don't combine them.
If you are falling for this girl and she feels the same then I say it's ok to be vulnerable and tell her your problem before you have sex. Most likely she will understand and want to help you. This would be awesome cause you could just practice practice practice :)
I used to even think about getting a hooker to practice with haha. But don't go that far lol finding a girl who you love might be the ideal thing for you. I don't have any sound remedies and I'm sorry for that but just keep having hope and don't beat yourself up about it. Girls would much rather you come quickly than not at all ya know? There are lots of guys out there who struggle to get it up and it's my opinion that that would be a much harder issue to deal with.
You are definitely not alone though, lost of guys have this. Just keep your head up, practice however you can, and try different things. You'll be fine I promise.