i need gf advice

if you don’t think you wanna read all this pls don’t, i just need to get this off my chest.
you guys i’ve been with my gf for about 3 years now and we’re going good for the most part but i realized something, her happiness has gone down severely and i feel like it’s all my fault, her sister passed away about 2 years ago and it ruined her and i know ill never be able to help her heal from that and honestly im not trying to because it is not my place but i still try to comfort her and im such a bad gf. and i see pictures of her before and she was just so confident and happy and had friends and i ruined that for her. i always let my emotions get to me and i ruin our days together and i always make things about me when im not as hurt as she is, ive been through things but all that shit is in the past it has no reason to effect me in the present but for her she’s going thru it as of rn and i suck at helping her. How could i better myself and actually do things i say, like becoming a better person for her, we’re both girls and i know girls are girls and will be emotional but idk what to do. i took her spark away and if i were to leave her, ill take everything from her, she quit school cus of me bc i wanted to do online, she has the same job as me so one of us would have to quit, i made her lose her only friend (that’s because of a whole nother topic tho, she doesn’t like when i have friends either so it’s a tricking situation but i still blame myself) and god i could never leave her everything of mine is hers and what’s hers is mine i just can’t let this go down because of me, and im self aware so i need to fix this but bro i dont know where to fucking start i’m doing good one day and fuck up the next