Hi Cheyenne,
First off, I'm so sorry to hear that you and your girlfriend have been going through such a difficult time. I know how hard it can be NOT to think about something, and to ignore what sounds to me like intrusive thoughts. (Keep in mind, however, that I am not a medical professional, just a writer with some personal experience in this area.)
Whether or not your dream was a nightmare, it does seem to me like it's probably tied to your recent thoughts and experiences around self-harm. Again, I'm not a professional, but in my unprofessional opinion, it seems like maybe the reason your dream didn't feel like a nightmare until you woke up is that, unconsciously, your brain may be linking self-harm with relief. Even though it made you feel worse in the past, now that you keep thinking about it, there may be a part of your brain that is trying to convince you that simply going through with it will make those intrusive thoughts go away. (In my experience, however, the more you engage in self-harm, the harder it becomes not to think about.) If this is the case, it may be that your dream felt pleasant because you were dreaming about that relief you're hoping for, but you were distressed when you woke up because you don't want to keep hurting yourself.
As for being triggered, yes, it is certainly possible that your girlfriend's self-harm may have triggered your own. This may make things difficult for you if she keeps self-harming while you are trying to recover, but it's not an impossible barrier to overcome. Ideally, of course, you will both be on the path to recovery soon.
I understand not wanting people to worry over you, especially when you're worried about your girlfriend. However, please try to believe that your own well-being is every bit as important as your girlfriend's. I'd like to assume that she wouldn't want you to suffer, especially in silence, any more than she would want you to. The same, I hope, goes for your parents.
Is it possible to reach out for professional help on your own? Some schools have free counseling services—if not, you can try calling a hotline that might be able to connect you to the right service provider. I know getting help can be scary, no matter who it is from, but as someone who went through recovery on her own for a long time before asking for help, I can tell you this—it's much easier to get better WITH help than without it. Here's our resources page:
https://www.healthyplace.com/other-info/resources/mental-health-hotline-numbers-and-refer…
However, I do think it might be helpful for you to talk to one (or all) of the people you mentioned. It's possible that you and your girlfriend could work toward recovery together, rather than suffering separately without talking about it; just take care not to use any accusatory language or point fingers in any way. You don't want her to feel like you're trying to force her into anything; rather, try to focus on simply letting her know that you're worried, that you care, and you want to help her feel better in any way you can. Or, you can start by talking about you--maybe hearing about your struggle will help her feel comfortable with opening up to you about her own. And yes, your parents will worry if you tell them, but they may worry even if you don't. Looking back now, I realize that I acted a certain way when I was self-harming, and even though I didn't tell them what was wrong, they knew something was wrong without knowing what--and that can be worse than knowing the truth.
However, disclosure is a very personal thing--so please do weigh anything I say here against your own judgment. You know your girlfriend, your parents, and your relationships better than I do; it's up to you to make this call, though I hope I've made these decisions a little easier for you.
If you have any more questions, comments, concerns, don't hesitate to reach out. I'll be reading.
Wishing you the best of luck with your recovery,
Kim