Firstly, really sorry to hear about your friend. A close friend of mines passed in the same manner and it's been a long road, but you will see better days again. Don't ever close yourself off to the wonders of the world on your harder days. Don't be afraid to ask for help either.

Like you, I had a period in my life where I practiced mindfulness a lot, and things were great, but I ultimately realized that I was just just sedating myself and avoiding. Please look up the word "spiritual bypassing"---most meditators and mindfulness practitioners are guilty of this. I wasn't processing my emotions and developmental tasks in my life with these techniques. They would emerge and I would just become 'still' and 'present'. I was calm, but I wasn't growing. I was just releasing and staying in the present moment, and i'm eating up spiritual books like candy telling me how 'thoughts' aren't me and that i can have freedom in the moment blah blah blah. Ultimately, I started to journal (which was very difficult) as everything began to emerge and it led to some very depressing bouts, but I started to constructively working through what was happening which paved the way towards real personal growth, but it's a much longer road with setbacks, and professional help through a therapist may be the best bet for many people. Mindfulness is just a tool, it's a technique from people who practice a monastic lifestyle in the East. Western life is fast paced and dynamic so the mind is going to jump around so you cannot depend on psychological crutches from other sides of the world to manage your mental health. Therapy is what is needed. Mindfulness will only help you relax when things are going relatively smoothly as is.

With my friend, I ultimately had to forgive him. I had to find compassionate for him, and then I had to let him go.

Good luck.