To feel angry and alone when my girlfriend died from an overdose

My Girlfriend of 6 years died on January 22, 2020 from a Heroin Overdose. Too make a long Story short we were driving to Dinner and she said she was Addicted to Heroin. For me I haven't done any Illegal Drugs in my Life so this was A Huge surprise. She said she wanted help so I called her mother for her and she went to rehab for 90 days. The thing was her Mom didn't want her living with her and she lost her apartment and she didn't have a place to stay after rehab. So after talking with my Dad who Lives with me cause he's 74 he agreed she could stay temporary. We were not on good terms me and my girlfriend at the time because of her addiction, but I let her stay in a room beside my room. She was doing Good she got a job and went to meetings. She started missing work and sleeping all the time and I was at work and got home and went upstairs and her light was on. I figured she was ok cause the Tv was on and I went to bed. The next morning the 22nd of January I went to the gas station and came back home and went upstairs in her bedroom and she was slouched over the bed and I went to too see if she was ok and I touched her and she was Cold and her body was like Hard. I checked her pulse, but there was none and called 911 and she died that morning. I'm ANGRY and GUILTY because I did everything I could for her, but it still wasn't enough. Now I'm extremely DEPRESSED,LONELY,SAD,LOST, and don't understand WHY she did what she did. Maybe I need advice from someone who has gone through that also because DRUGS have never been a part of life and she never told me till 4 yrs later in the car. I lost my BEST FRIEND FOREVER and feel like I should have died instead of her. I guess that's the GUILT talking. I'm DEVASTATED.😥😥😥