"Telling her straight out how you feel" is communication. Telling her what to do, and what will be the consequences for a certain action, is setting an ultimatum. In other words, an ultimatum includes, by definition, some variation of the concept "do ______ or else I'll ______."..
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If the emotional turmoil of the relationship is too much for you, by all means tell the girl. She deserves to know, and perhaps the implications will be enough motivation for her to get her poop in a pile. Or perhaps this just isn't the right time for you two to be together, in which case there's no point in remaining quiet about your misgivings. ..
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Be kind -- she's clearly feeling a little fragile and confused these days -- and leave the door to a common future open, or at least ajar. She will appreciate the gesture even if it doesn't work out for the two of you...

It's not an ultimatum -- it's just sharing...
Look, if you want to be in a committed relationship, and she doesn't. Then each of you have a choice to make. Guys very understandably get tired of drama and girls changing their minds a lot. Tell her to take some time and think about what she wants and get back to you. Give her a week or two to think about things. If she wants to go slow and not be committed right now, then that's fine. But she needs to commit to what she wants and not put you on an emotional roller coaster. It's understandable that she wants to keep a close watch on that heart of hers -- it's been trashed in the past. If she's not ready, she's not ready. But tell her to really think about it and get back to you with an answer she can stick with for like a month or so -- so you don't go insane...

I can only imagine how hurtful it must feel to hear something like that but I have a friend who on occasion says to her fiance that she's not sure if they should really be together and gets scared thinking 'maybe I'm doing the wrong thing?' then she snaps out of it...I told her that if my boyfriend said anything like that to me I'd kill him!..Then she confessed that her parents were together until she was about 25, had a lovely happy marriage while she was living at home and then out of nowhere (it seemed to her) they just ended it and this is where her new fears have come from...
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Now I know this isn't the same thing but possibly her former relationships have left her a bit insecure and this could be why this is happening...
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I hope you get it sorted!..

I definitely think you need to tell her how you feel. Give her a week or so to figure out what she needs and make it clear that you can not take her wishy-washy behavior anymore...The key to any relationship is communication and if you guys can not communicate your feelings then the relationship may not be right to begin with...

If you really care about her, give her some time to heal her wounds some more. It's hard to find someone special who returns your feelings. It sounds like she is mostly on board with you, but afraid of getting tossed overboard again. But, like the others said, do gently point out the affect this is having on you. I imagine her behavior might make you more wary about relationships, so it's like she's unintentionally creating more of the problem she's having herself...