"Widowed" cat will not stop crying

Hi,

Until 2 months ago, I had 2 wonderful older cats. The 15 year old
male was finally put down when his cancer was too much for both of us.
My 17 year old spayed female cat, who has been left behind, now howls
deep gutteral meows on and off all day and night. I thought after a
few months it would improve, but it has not. I go to find her and she
will be sitting on the edge of the counter howling with all of her
might. I will pick her up and cuddle or take her back to bed, and she
will purr and be fine, but within an hour, she will be at it again.

I know she has been traumatized by losing her friend, but I just don't
know what else to do to console her. Does anyone have any
suggestions? Love and treats just doesn't seem to be working and I
have no interest in bringing another cat into the house as a
replacement. It is obvious she is miserable, and I hate to see my
sweetheart in so much pain.

Thanks in advance to all responders - I am at a loss.