My son committed suicide and left me a dark letter

My 20 year old made the choice to end his life.... I came in the house and he had overdosed on my Prozac. He left me a letter that I didn't even find until after I got back from the hospital. The worst part is he died in the ambulance, so if I would have gotten home earlier I might have been able to save my babies life. My "boyfriend" showed no emotions about my son dying. While I'm crying telling him to get up here to the hospital he is just saying he's busy. When I finally got home I found the note he left in his bedroom. I'm not gonna day everything in it, but he told me my boyfriend had been sexually abusing him since he was 10. I had a panic attack reading that and when my boyfriend asked what was wrong I asked him about what my son said he starts fumbling all over his words and then says "I mean how can I rape a 20 year old?" He did not look at me once. I packed my stuff and left. He begged me not to go and said to my face "Who are you gonna believe. Someone who loves you or your DEAD son". I shoved him and left. I'm at my mom's and I cannot stop bawling. I lost my son and lost the person I THOUGHT my boyfriend was. I blocked him on everything.