@Birlngsnotnicepeople I totally understand where you are coming from but...
Do you think that because your adult children don't visit/contact as often as you would like, you must have done something "wrong"?
Could it not simply be a case, as @PivotPivotmakingmargaritas mentioned that they're wonderfully independent, busy adults, making their way?
You've mentioned that "I am a bit lonely and we are an afterthought"
The loneliness, is for you to rectify and not for your children to fulfil. The afterthought comment is somewhat concerning as you seem to think that your children are somehow failing your need?
Your "expectation" of what your relationship should look like, does not align with theirs.
So a change in your mindset could be useful and bring you the most peace to enable you to just enjoy their company, when you do actually see/have contact with them.
Don't be sad @Birlngsnotnicepeople, it is what it is 🌻