"I'm trying to remember what's good," Tequila told PEOPLE in an exclusive interview Monday. "She was such a loving and beautiful person. I want people to remember her as that."
Though they had been dating only a month, Tequila, born Tila Nguyen, says she and Casey had known each other through mutual friends in Hollywood – and initially didn't think romance was an option.
"We just hated each other through gossip. When people talk about someone you have preconceived judgments," Tequila says. "But one day I texted and she was the cutest thing ever. She was nothing I had envisioned. I thought, I cannot believe this is the Casey Johnson people used to tell me about."
Nervous First DateOn their first date, "She was so nervous at my house. Dropping things. I could tell she was nervous ... We just ended up laughing all night. We just fell in love," says Tequila.
And they didn't waste any time moving the romance along. "Lesbians tend to move very fast. They call us U-haul lesbians and that's what we did. She U-hauled herself into my house the next day," says Tequila. "She brought all her dogs. Racks of clothes. I have two bedrooms and she filled up the entire bedroom and guest bedroom with all her shoes and clothes. I came home and the bathroom is filled with all her stuff. Makeup, picture frames of her daughter Ava everywhere. I was like this is hilarious. I said I feel like we'd been married five years."
Indeed, marriage soon became a topic of discussion. "One night she comes over and she was so nervous. I was like what's wrong with you, calm down!" Tequila recalls. "She was so nervous and so cute and finally she pulled out this fat ring that I had never seen before."
After presenting Tequila with the 17-carat diamond, the Johnson & Johnson heiress said, " 'Will you please wear this? I want you to be my wife.' " Tequila recalls. "We started laughing and crying."
Starting a Life TogetherThough Johnson maintained another residence, where she was found dead on Jan. 4, "We were so excited about starting a new life together," says Tequila. "I was going to buy the house and she was going to buy the car."
Tequila also had plans to formally adopt Johnson's daughter Ava, 3, whom Johnson adopted from Kazakhstan in 2007 and who had been living with Johnson's mother Sale for several months.
"Casey and I were working out [getting custody]," says Tequila. "I was going to adopt Ava and we decided she would call her Mommy and me Mommy T. We were going to have a big family. She wanted seven children, but she said she wanted me to be the pregnant one."
Shocked by DeathJohnson's death from as yet undetermined causes came as a shock to her fiancée. After squabbling over Casey's untrained dogs in a phone conversation on Dec. 30, while Tequila was in her home state of Texas visiting family, Tequila says she went a whole day without hearing from Johnson.
"At first I thought sometimes she likes to sleep in till 5 p.m. She has done that before and it freaks me out. Then I thought maybe she was mad at me. We'd fight over little things. Then she'd apologize. Or I thought she was with the Hiltons."
After calling friends and an assistant to check up on Johnson, Tequila learned the terrible news. "I booked the flight home right then. I was in shock."
Tequila says it was only days after the news broke that things really sunk in. "It really hit me when I found all the cards she had written me," she says. "My therapist put me on suicide watch. I've been getting help but I'm fine now. I know Casey wants me to be alive and she hates seeing me like this. She always hated it when she would see me cry."