I've done this loads of times now - similar situation to you. If you wait for everyone to be ready to go on holiday with you, you'll never get to see anything. Live's too short for that!
I did a safari three years ago with:
They have loads of activity based trips, some of which are specifically catered to solo travellers. Try and choose a trip that doesn't shout 'honeymoon'. So, if you're looking at a safari -- for example -- go for the wild camping option rather than the luxury lodge one. Am sure the same will hold true for other types of trip too.
Last year I did a trip up the east coast of Australia on my own. If you decide to do something like that, rather than an activity based holiday you'll probably find you spend a fair bit of time on your own, especially in the evening. I tended to make sure I ate a large meal at lunch, did /something/ until late in the evening and had something light for supper. That way you're not sitting on your tod too long in a crowded restaurant.
You'll often meet people in a similar situation whatever type of trip you do. Have made plenty of new friends going on holiday on my own which has opened up opportunities for future holidays too.