My aunt by marriage often goes on cruises. She used to go with her husband and, when he died, she plucked up the courage to continue to go. She tends towards the smaller liners.
My friend has just been on a walking holiday, joining up with a group of other singles, and seems to found it friendly and to have enjoyed it.
We recently went to Shetland and came across several women travelling alone in campervans. The small campsites are quite friendly, so they had that support if they needed it.
So - have a look and see what you fancy doing and give it a go. Make it a short break first, if you're nervous, to see how you feel about it.