Dating someone after they lost their former SO

I’m currently in a relationship with someone who is pretty much my best friend. Not too long ago he got into an incident which caused him to be critically injured and his former significant to pass away. What I’m worried about is the fact that they had a long lasting relationship where she left a great impact on him and those around him I don’t want to sound selfish but I feel like I will never measure up to anything. I’m really anxious about this relationship cause I really do love and care for him but I feel like we would never work cause of my insecurities. What should I do? I have also spoken to him about it like telling him I do not want to rush into a relationship and have him not be ready mentally, emotionally, etc... or be like the rebound or replacement. At this point I don’t know what to do I feel like I have no right to feel this way. Can anybody offer advice? What should I do? How should I approach this relationship?