I dont know how to be tactful.?

I think you have to change that one from the inside out...
Speaking the truth as you see it is a very noble quality and it sounds like you have that part down pat...Kudos to that...Now maybe you just need to learn to view the world around you in a better light...Then when you speak your version of the truth it won't sound so bad...
Reality is after all 90% perception. If I am living well I am in a head space in which I see what's good before I see what is bad, which is decidedly more difficult some days than others. It requires work, but it is usually richly rewarded...Good Luck To You Though:)

Well you already recognize the behavior which is the first step. The next step being able to recognize the situation when it arises and change your behavior accordingly.

the only thing i can think of is maybe thinking about how what you say will be effecting the person you are talking to. sorry i cant be more helpful!

I tend to put my foot in my mouth quite often. It happens when I'm in a hurry to speak and don't think about how what I'm saying sounds to someone else. I just needed to slow down. Actually I think the bitterness you say you feel may be a bigger problem on the job. People will sense that from you and it will affect how they treat you. You might want to work on the source of the bitterness and ridding yourself of that. Good Luck.

It helps if you learn to stop and ask yourself how you would like to hear the same information.

I've been working on this one all my life. Eventually, no one wants to hire you. My maternal grandfather had to take up farming because he couldn't behave. I think it is genetic. I suggest you cut your losses and run. Take up an occupation where you are by yourself: sheepherder, truckdiver, short-order cook in a small restaurant (not too small, a waitress is needed to isolate you from the public), small aircraft pilot, etc . . .

I've had trouble with this too. I'd try to pay attention to how others say things at work, that you would have been more blunt about, and try to pick up their cues and tips. Until you've picked up these skills, I've learned it's best to simply bite my tongue. And don't be tricked by anyone saying they want the truth: usually they don't (from blunt people anyways). For example, someone states a plan that I feel will not work (personal, not work related), yet they haven't proven to be ready to hear the truth, I say "that's an idea" or "good luck with that". If it's work related and I have a better plan, I'd say something like "or maybe option B". It sucks to have to beat around the bush (or B.S.), but that's how most people appreciate things. Also, if possible, at work limit impromptu communications and opt for written/digital communication. This way, you can think of how to say what you want to say instead of being put on the spot.

Treating others as you'd wish they treated you is a start. Are you tactless because you put yourself over them?

To me, the problem is learning how to lie. People call it 'white lie', but some folks just aren't good at it.

you've gotten some good suggestions here, and some 'tactful' ones. Good luck.

Read a book called Fish. It is about having a good attitude even if you dont like what you are doing. It is short and you can read it in less than an hour, but it has a great message.