My ex girlfriend of 18 months was a drama queen; she'd change her mind constantly, have emotional outbursts. She never worked, wasn't in school, nor very busy at home as I did most of the cooking and cleaning.
      Then, I discovered she was cheating on me (I never cheated), and she thought nothing of it. I ended it, but the relationship left me stressed, exhausted and ate up much of my savings.
       Today, I'm in a much better relationship, we have plans for a future, we're both hard workers and get along great. Nevertheless, I remember the old days and sometimes I project this on my girlfriend. I hate to do it, but can't help it. I'm worried it might affect our relationship in the future.
      - Anticipating

Yes, you CAN help it! You've already started by recognizing the possibility of messing up. Now, get rid of the relationship labels and think only of the individuals by name - as in, "Sally" was a lazy cheater, "Jen" is a wonderful partner. 
      You've already learned the lessons of the past by moving on. The only element in this situation that relates to the previous one, is YOU. So, think about what you might've possibly contributed to things going wrong with Sally and how to keep them out of play this time.
      Example: Consider whether your contributions were, 1) bad judgment in choosing Sally, and 2) insecurity in hanging on despite obvious problems. Hopefully, your judgment is now much better and you now look at any couples' issues that arise with the confidence to communicate and problem-solve.
      Get it?

We've lived common-law for more than 20 years; I wish to be LEGALLY married so that his estranged children and ex-wife don't try and get at his estate should he die suddenly (he has a few health problems, nothing serious). But he drags his feet on getting married, though I've said I feel very vulnerable.
      He hasn't shown me his will and I have no idea where I stand; even the house is in his name. I now have a grandchild in another city and enjoy visiting weekly when I can. He can retire soon with a good pension, our home is paid off and I've suggested we buy a home there.
      Yet he won't even accompany me to visit the grandbaby, although I ask every week. He doesn't like change, and may not embrace retirement fully.
      I'm eight years younger, have taken early retirement with a small pension and he considers me an indentured housewife. He's financially astute and thinks I should be working full-time again. I'm thinking of leaving him.
      - Unhappy

You're at a crossroads and he needs to hear this: It's either Transition Together or End of Era.
      Like everything else in a partnership, a "transition" calls for compromise on both sides. You want to marry because you feel vulnerable; if he doesn't want marriage he needs to reassure you of future security, for example, by showing you his will and its provisions for you.
      OR by buying life insurance on himself with you as the beneficiary (talk to an insurance broker about how this works). OR he explains why he's against marriage in a way that you can accept.
      If he does none of the above, you need a reasonable explanation why not. Plus a private talk with yourself about where you want to be five years from now. With him, despite the differences? Living the Grandmother life elsewhere, but on your own?
      The choice is yours.

My future mother-in-law's smothering my fiance and me and ignoring our needs. She calls us 24/7 with stupid stuff like, she saw her son's initials on someone's license plate. We're stuck sharing a car - mostly her fault - that she uses to get us to do what she wants.
      I get headaches when I think of her or hear her mentioned. I'm afraid to answer my phone. We've broken up previously because of her. We tried to set boundaries but she ignores them.
      - Wit's End

Stop "sharing" the car, even if you have to be inconvenienced. Tell her, reasonably, when is the daily time that you can answer her call and don't pick up at other times (but listen to her message in case of real emergency).
      Recognize HER needs, too. If you don't give her some attention she'll naturally keep seeking ways to be part of her son's life.