I’m so sorry for your loss. In my experience the sadness doesn’t subside, but you get stronger. I lost my Mum 5 years ago and have moments where it feels agonizing. I try to hold on to the lovely memories and how lucky I was to have her. You can access some therapy sessions for free at the GP if you think it will help.
What helps me; Remember how much love you showed them. How happy you all were when you spent time together. How they would be cheering you on right now. Take notice of fabulous things about yourself that were passed down to you from them. Cry when you really need to, then think of them and smile.
All the best

I am so sorry you are experiencing this, loosing someone you love terribly is so hard. It is natural to think about these things, it can take years if not a lifetime to get over someone passing. Have you considered speaking to a professional to help you work through some of your sadness and thoughts? It might help you manage it a bit better.

It can take years to give the loss of a loved one a place.
I think that the fact that you had to out both your mum and dad in a nursing home due to dementia, adds to your grieve.
My mum suffers Alzheimers too and it hurts to see her deteriorating
My niece is diagnosed with PTSD after first losing her mother (my sister) then her brother (my nephew).
Have you considered grieve counseling ?

I’m really sorry you are going through such a difficult time. I just lost my brother recently and am struggling too and not sure how to navigate forward. All I know is, everyone’s different, everyone grieves differently and there are no expectations of where you need to be in your journey. Be kind to yourself, seek help and support and do things you think your loved ones would have been happy to see you do.

Just remember to be kind to yourself , there is no time limit on grief and see the sadness of a reminder of how loved you must have been.