3 years ago my grandmother became unable to stay in her home alone any longer. Her children were either unable or unwilling to take her in. I was living in another state and moved my son and I in with her to take care of her so she didn’t have to leave. 6 months later Covid hit. If she’d been in a home, she probably wouldn’t have made it. 
The first two years weren’t too bad, she is legally blind, has dementia, and had some dizziness. In exchange for 24/7 care, I paid no rent, but we split all bills. I also had restrictions on who I could have over. Last summer she had a stroke and things got significantly worse. I sent my son back to the other state to live with his dad to go to school. 
I admittedly had some issues on and off with my bipolar. When I was depressed, I didn’t get much done, but I tried my best. Last October I tried to work from home. In 5 months, I ended up hospitalized. Right after I was diagnosed with an aneurysm and had surgery. And also during this time it was determined my grandmother must be with someone 24/7. 
I’ve decided to move back to be with my son. My grandmother is now 95, has had a heart attack, and needs care I can’t provide. 
This is where I need advice: my cousin who lives across the country has gotten involved in setting up appts to look at homes for my grandmother. I was at my moms and saw there was a group chat full of information and pics. I was hurt that I, of all people, had been excluded. I didn’t want anything said but my mom asked why and my cousin said, in the group chat, that I expressed no interest and my mom could forward what she wanted. My sister is on this chat so it’s not just aunts and uncles. After my mom mentioned this, I was still not added. I was hurt and sent a message saying so, and that I never said I wasn’t interested…to the contrary. That I knew her better than anyone and would have loved to give input. I was not accusing nor mad. 
My cousin came at me hard. I had to take responses to just her and I. She said all sorts of things that weren’t true, called me delusional and wouldn’t stop going off on me. I finally had enough and told her to f#%k off (which was wrong of me.) Her response was “You f#%k off you lazy user.”
I was incredibly hurt and betrayed. I have never used my grandmother. If anything I was used for all the money I spent, all the 24/7 days I spent when that was never part of the agreement. If I wasn’t there, a full time nurse or nursing home would have cost way more than me renting a bedroom
But it hurt because I did have a period where I wasn’t my best. And I’m hurt anyone would think I’ve used her when I gave up 3 years of my life when no one else would. 
My question is: I did swear but I didn’t call her names because you can’t take words back. How can she come back from that? My  instinct is to never have anything to do with her again. Am I over reacting?
For context, my son &amp; niece think she’s a bitch and my mom and sister say you have to walk on egg shells around her. I’ve always tried to defend her. But not after this

LD:TR Gave up 3 years of my life to take care of my 95 year old grandmother. Now that I’m moving, I’m being excluded from information regarding her. When I mentioned that I was hurt by that, by cousin called me a lazy user, which is rude and hurtful but also triggering because of my mental issues