Sorry, gonna be a long text. So, my gf(21) and I(21) have gotten together in Oct 2019. We were together 2 years before that but it wasn‘t a serious relationship and only lasted 2 months. She had always been a little sad because of a bad relationship with her family, depressions and sometimes panic attacks. I tried to be there for her and sometimes it worked but things lile a bad relationship with the family just can‘t be fixed and is still prominent now, 3.5 years later. I sometimes couldn‘t handle her emotions since I‘ve always been relatively cold and rational while she‘s a very emotional person. Because of that and some other disagreements we had a very rough start but got through it all. Here comes my biggest mistake I ever made. I had a female best friend at the time and was planning to break up with my gf since i wasn‘t happy anymore. I wanted to talk with my best friend about it since i didn‘t feel like talking with my gf was an option/wouldn‘t work. She begged me not to meet her and i said i wouldn‘t because i couldn‘t see her sad. In the moment i didn‘t realise how sad she was and i wanted to talk with another person so i still met my best friend and talked with her. She talked me out of breaking up and i talked with my gf and told her that i met my friend even though i told her i wouldn‘t. I didn‘t realise how much that would hurt her at the time since she seemed happy that i decided to stay and we continued the relationship. over the next couple years I‘ve been an asshole whenever she tried to tell me how she feels and just now i‘m starting to realize how much damage i‘ve done to the relationship and to her mental state. Of course i try to be a better bf and all but i feel lile no matter what i do, i doesn‘t make a difference because that one thing in our past just overshadows everything else. I never listened and am only now ready but feel like i lost her already. She always tells me she loves me and we have a good time together but whenever we get into a discussion which is sadly quite often she tells me how unhappy she is with my behaviour and most of that comes down to that one moment. I never understood why she can‘t get over it and just forget about it since it‘s been a long time and we had other, bigger things that she did eventually forget. Now i understand that i basically traumatised her and even if it‘s a small thing for me, it meant the world to her since that was the only time i ever lied to her. I broke her trust and haven‘t been able to rebuild it no matter what i tried. I know that she doesn‘t understand a normal sorry since it‘s not her love language, she likes to got our and eat something but i don‘t have the money to do that atm since i‘m studying and need most of my money just to drive to work etc. i like to cook for her but that doesn‘t solve anything obv. i feel like i tried everything and nothing works, this is my last option. how can i help her with the trauma i caused?

please help me, she is the love of my life and the woman i want to marry someday.

TLDR; I‘ve made a big mistake 3 years ago and need to help my gf with the trauma i caused.