The sadness just takes time. Being single is fabulous when you get into it - trust me - I was the solidest married woman in the history of married women, but I ditched the bugger because it had to be done, cried buckets, drifted around like a lost thing for a couple of years, on ADs, the lot. And now? If the millionaire of my dreams proposed tomorrow (unlikely), I'd hum and haw and suggest we live in separate houses.
The bitterness, though - I think that comes at least in part from you not believing you are worthy of a successful relationship. It's really not a matter of worthiness. Some pretty ghastly people somehow seem to find a life partner, whilst others are amazing and gorgeous but just never get together with the right one (just think of some famous Hollywood actresses: beauty, fame, wealth - and dumped). You are just as deserving - or undeserving - as both the best and the worst of them. Or to put it another way: we don't get what we deserve in this life, we just get what we get. It's not a fair universe.
How you behave does make a difference, you could I suppose be putting suitors off without meaning to, but again that's not always the case. You get wonderful wives (or husbands) who never put a foot wrong but the guy (or gal) goes off with the secretary/neighbour/dustman just the same, and that's often down to something lacking in the one who leaves, not the one who is left. Some make a great show of wanting to settle down, but they get bored, not because the other person is boring but because they themselves crave novelty. So you think you've found the one to spend your life with, and they may appear to go along with it at first, but find they can't hack it after all. There's nothing you can do about that. Human beings can be fickle. Honesty from the start would have helped but that's another rare commodity.
As for the would-be kindly people asking whether you're dating - KILL THEM. Er, I mean, practise brush-off lines such as "I'm far too busy for any of that at the moment, if the right one comes along they come along". If you get into "I've sworn off the opposite sex for ever" they'll start arguing with you or digging into your whole life story (which is none of their business) and you'll end up feeling like shit, as you know. Keep it light and don't let the bastards grind you down.
Meanwhile, just keep putting one foot in front of the other and eventually the sun will come out again. Unless you're clinically depressed, in which case you may need a helping hand in the short term. Don't be embarrassed if you do need help; it's just an illness like any other, no more shameful than a broken leg. You don't have to be perfect. You're human.