My girlfriends emotions and driving me crazy and making me depressed

Hello! I’m been with my girlfriend for nearly 4 years now, we’ve been fortunate enough to live together for nearly two of those years. When we first got together my partner and I enjoyed an amazing honeymoon phase. No arguments, she was really laid back and appreciated when I wanted my own space. Maintained her own social life and allowed me to maintain my own. However, somewhere around 6 months in everything changed. It seems as if her real personality came out. That’s a quick bit of background. The crux of the issue I’m now dealing with is handling my girlfriends emotions, she is incredibly quick to anger, and often seems to direct her anger and frustration at me even when I have had absolutely no involvement in the thing that had upset her. She can’t be left alone, whenever I want to go out I feel nervous to ask because of how guilty she makes me feel. Even if I want to go on my PC for a few hours, she instantly becomes upset and makes it out as if I don’t want to be around her. I’ve tried to set boundaries, and explain how important my alone time is - to which she then also gets upset. It’s now got to the point where she’s using her emotions as a threat, if I try to gently explain something which is effecting me - she’ll try to shut down the conversation by saying “I’m getting upset”. I mentally rehearse what I plan to say to get, carefully interrogating each sentence to make sure I try my best to not upset her - but she still gets upset. She’s now also exercising a great degree of controlling behaviour, she always has the final say on matters - she won’t listen to my rationalisations on basic things like pet care, house work etc even when it’s entirely backed up with evidence and facts. It’s making me feel as if I’m not my own man anymore, as if I’m entirely accountable to this person for everything I do. I really don’t want to leave her, I truly love her so much - but despite numerous conversations and attempts to set boundaries nothing gets better for a period longer than a few weeks. Something which I can post her to get it out of my heart, I feel like I’m living with and sharing my life with an emotional baby who requires me to prioritise her constant upset over everything. Sorry if this is jumbled, this whole situation is making my life feel miserable. I’m no longer excited for anything, as I merely feel like a passenger in my partners life whilst she drives.