I’m so sorry to hear about the death of your ex. It is natural to grieve for him, as he was once a big part of your life, and was still your friend. It sounds as though you are feeling very isolated and that you can’t grieve openly for him. I’m glad that you’ve found this site, as it’s really important to have somewhere you can share those feelings.
Here are some other posts by people who have lost an ex - these are a bit older, so the posters might not be active on the site any more, but it might help you to feel a little less alone in what you are experiencing:
Death of ex wife
Loss of an ex partner
Does it get easier
This article about Disenfranchised Grief on the site What’s Your Grief? talks about the difficulties of grieving a loss that it is not widely understood or accepted that you’ll be grieving for - with losing an ex as one of the examples covered.