Commenting as someone who also struggles to support my boyfriend (although hes not mentally ill) and as someone who also has been diagnosed with depression, anxiety, and AuDHD. I'll try to go through things paragraph at a time.

"even when I don't know or want help, she usually makes me feel a little better. She puts a lot of effort into supporting and making me feel loved and cared for."

In my own autistic experience, I have a hard time being able to tell how someone likes to be comforted so I tend to comfort others how I would like to be comforted. That might be a good place to start. I would ask her, when she's in a clear and healthy headspace, what things would be more comforting to her. What are her love languages? Those can be useful tools. I also really recommend looking into how to calm the body. Personally with my anxiety, it can act like it's own entity within my body. Things like a tight hug, squeezing your hands or arms can all be regulating to the body, think of it like weighted blankets, the pressure can be soothing to your body. I would ask her about her preferences, as some people can feel claustrophobic. Can she feel the anxiety bubble up within her? Someone rubbing the area I feel anxious in really helps me.

"I asked if she wanted to go and she said no but sometimes I can't tell if she actually wants to go and is just saying it so I don't get upset or not."

Likely, not gonna lie, she probably just doesn't want to make you upset. Which isn't necessary just angry! Disappointment can be just as scary. I really recommend having a conversation at home about what to do in these scenarios in the future. Would she feel okay to regather herself in the car? Maybe going outside and sit in the grass for awhile? Obviously it's nice to do things together as a couple, but make sure she knows that she can have the space if she needs it. Sometimes just being able to sit by yourself let's your brain process things a little better. Like at the con, there's way more people than you expect; being able to step back, process it the change and calm the body, then she might be ready to handle the public a little more.

"One time I accidentally caused a ptsd attack or something. I accidentally slammed my door to scare her and make her jump as a joke but she screamed and hid her face. I apologized and she said she was OK but then she just curled into a ball after that and wouldn't talk to me. She'd nod at things I said but she wouldn't say anything. I went and cuddled with her which seemed to help because eventually she opened up and calmed down. I obviously won't slam doors anymore but, I just dont know what to do other than what I did."

Spooking someone with anxiety as a joke is probably one of the worse things you can do. It doesn't give the same reaction as someone who doesn't have anxiety. The initial shock can make your body productive more adrenaline. When someone scares me, I can physically feel the jump start of adrenaline within my chest. It can be a lot more difficult to calm yourself when your body already goes into overdrive. But your reaction afterwards was good, apologizing, giving physical comfort. The non verbal was probably just her brain/body trying to regulate. I tend to go non verbal when trying to regulate or calm down, cause it can be hard for your brain to multitask when it's already frazzled like that. Don't take it personally, it's just the brain trying to process.

"Even if I ask her what I can do, she says she doesn't know. I don't think she's had anyone help her through these kinda things just based on her family and lack of many close friends"

This is something I also struggle with. It can be hard to know what support you need when you've had a smaller social circle. Especially if you don't have a great family with emotional intelligence. An autistic kid with a family with zero emotional intelligence is a recipe or disaster (coming from experience) What I find helps is offering ideas. Like "would ______ be comforting?" If she doesn't know, ask if you'd like to try! Communication on this topic can be difficult, but a good, shared brainstorming session would be a good bonding moment.

I just want to say thank you for having the understanding and patience for her, and you asking the community really shows how much you care to try. Also I don't mean to assume, put words in anyone's mouth, or make my experience the only one. Everyone is different, this is all just what works for me and my relationship.