First off I just want to say that electrical engineering is hard as fuck and you're a pretty good writer so you're clearly not stupid. I don't think you really think you're stupid but it seems like she's said a lot of hurtful things in that area. Skip to the next paragraph I'm just rambling about me here I entered a 3 year electrical engineering course when I was 15 but I quit after only 3 months. In my country instead of high school we have something called gymnasium which is a 3 year preparation for whatever you want to study in university other than that it's basically just high school. I just couldn't do it, it was too hard and I was very depressed and I couldn't lift myself out of bed in the morning. I didn't understand why I was feeling like that and I still don't but it ended with me staying in bed for 2 weeks and then checking myself into a mental hospital. Even though I quit so early I still somehow got an A in PLC Programming even though we weren't even close to finishing the course after only 3 months. The next semester I signed up for another school and that grade helped me get in to the school I wanted to go to. But enough of me. That was highly irrelevant and I don't know how that could possibly help you and I apologize for it.
If the veterinarian thinks your dog is fine I think you should trust them. Also based on what you've said it sounds like your girlfriend is abusive and manipulative. It doesn't sound like healthy relationship at all to be honest. What I want to ask you, if we just play with the thought a little now, if you wanted to get away from her where would you go? Can you stay somewhere else? If you are in an abusive relationship it's important to look for some other options just in case. For both of your sakes. Maybe you just need a little break from eachother, who knows. Maybe you just need to go talk to someone as a couple so you can get a chance tell her how you feel and have her listen to it without her throwing a fit. But right now I think the most important thing is that you look for a way to distance yourself from her just in case, then if things get even worse you atleast have something to lean on and somewhere to live. In a situation like this sleeping in different rooms is not enough.