You have a good understanding of your problem and what you have read is good advice.
I had the opposite problem, premature ejaculation. What I needed to do was to retrain my response. The longer I delayed ejaculation the less sensitive my penis would get and the longer I could last. You need to train your penis to be more sensitive. I would recommend that you don't masturbate for a few days and then when you resume use a lot of lubricant, be slower and softer and take your time. I prefer to use sexual fantasies instead of porn when madturbating. When making love, I think about all the things I like about my partner's body and their response to being pleasured.
Since I have gotten older I'm not as hard and can't orgasm every time I masturbate. The lesson learned is just like you can tell when you are going to orgasm, you can also realize that you won't be able to orgasm. Continuing isn't going to improve things. Stop and try again later.
Movies show some passionate kissing and then simultaneous orgasms. This is isn't the real world. Making love is both partners pleasing each other but it doen't have to be at the same time. My sexual soulmate and I would have simultaneous oral sex. At one point my partner said if she could stop what she was doing so she could concentrate on her own orgasm. She enjoyed performing oral sex on me as long I wasn't performing it on her.
When your girlfriend tried ot give you an orgasm, was it after intercourse? You may have been at the point when you were able to orgasm. If this was the case, see if she can give you an orgasm before intercourse. Giving your partner pleasure makes you feel good, doing something nice for someone.