How to make it work this time- And how to ask her out again

So there's this amazing woman. I've been blessed enough to know her for nearly three years now. Some of that time, we've been partners. Some of it we've been enemies. Most of it, we've been friends. Fourteen months ago, we made an agreement we would put our feelings for one another on the back burner, so to speak, so that I could get my ducks in a row and figure things out. Fast-forward to present day, and I've got things pretty well figured out. I am in a much healthier place emotionally then I have ever been. I have my issues, but I work terribly hard to overcome them every day, and each day I am working towards a happier, healthier future. And it's come to the time where I feel I may finally be ready for something with her.

For a long time I avoided talking about things with this woman. For one, she's often very busy. For two, we're involved in a rather complicated relationship situation in which I have long-standing resentment and anger towards someone she is extremely close to, and I struggle with that sometimes. For three, I'm terrified of getting things wrong. This would be the third try. We both know that this is the last try we have in us, and although I am so much better and know deep down I could handle it, I worry I won't be able to. I worry I will start to feel really jealous again, or the issues I have with her other girlfriend will really push us apart, and I worry because I over-compensate sometimes for how I treated her in the past and that puts pressure on me.

Lately we've been spending more time together, though, and every time we get close again it's just like two falling stars: we crash into each other. Our feelings come back to the forefront and we start realizing how much we missed one another. I think that the inevitable conclusion is that we will come back together again. The question is just when, and she is waiting on me to make the move, since she has been ready.

I was talking to our mutual boyfriend and he was telling me she says she misses me and wants to spend more time with me and "wants me back." I want those things too, but I'm just worried. He says that I should ask her on a date sometime. I'm just not sure how to proceed. I'm not sure if I should. I want to, so badly, but things need to be right. I need to be right, and she needs to be right. So how do I know, and what should I do?

[Eternal]

Well, as you know things won't work out unless the problems that caused your relationship to fail the other two times has been fixed. In this case, it's her friend that's been causing problems. Perhaps you could try talking to your friend about it, and see if you can both agree to just not bring up her friend in conversation? If she remains out of the picture, then you can just focus on your relationship.
It sounds like you really do want to get back together with her as well, and honestly if you've lasted 3 years of having some form of contact, then I think there is a chance that it could work out. You should definitely start by taking things slow though, and if you realize that it might not work out again, you can get out of it before things get messy and you lose her. It almost sounds to me like you're already acting like you're back together, you just haven't made it official.
If you do decide to give this one last try, you should also try and just completely reset and leave the past behind. You've both changed each time that you've tried this relationship, and so past mistakes shouldn't make a difference in the present.

So to answer your question, there isn't really a way of knowing if things are right unless you give it one more try. If you don't take the risk of trying this one more time, then you'll never know if things could have worked out. As for getting jealous, I don't know the situation with your friend's friend, but she obviously wants you and not her. And the fact that she wants to try this for the third time is pretty good evidence that she really likes you and wants this to work.