This post isn't gonna be pretty, so if you don't want to be in a bad mood right now, you may want to skip my post. Though, you have the opportunity to save yourself from a world of hurt later if you did read it. I'm going to make many, many assumptions here based on what I read between the lines, so if I'm wrong on much of this, I apologize.
I'm sorry, KnightGhost, but from the looks of it, you're in a bad position that might not be saved, and could easily get worse. If she's going out with another guy - or to be direct and blunt, if she is regularly intimate with him - while keeping you on the hook and intertwining with you emotionally, you are in deep trouble. If the boyfriend is a PoS who treats her bad, yet she continues to be intimate with him, it's because she's simply that intimately attracted to him. Moreso, apparently, than she is to you, if you've never been intimate with her. And after 5 months, you should have had plenty of opportunities to take it to the next level.
At this point, you could try to 'confess your love,' but before you seriously consider that, consider this: What have you done to be competitive at her boyfriends' level? Have you been overtly flirting with her with sexual overtones? Has there been any sexual tension? Can you think of anything that happened or has been said that might make her curious or excited about you intimately? How comfortable are you two with casually touching each other? If none of this is there then she probably considers you, effectively, asexual. And make no mistake, sexual tension is the basis of romance. Without any sexual undercurrent, you are a friend. Just a friend. Her PoS boyfriend is more romantic than you if you haven't been competing on this level at all.
Your success with gaining her romantic attention is low if you keep doing what you've been doing. If things do not change within less than, say, two weeks, I suggest phasing her out of your life since, judging by your words, you probably won't be capable of letting go of her emotionally while attempting to be platonic friends. And I know that's not what you want.
All of this is a time tested, oft repeated, tragic, reliable pattern. Hard though it may be, it is unfair that she is stringing you along, even if she is (partly) unaware of her actions, and their consequences. Of the two alternatives presented(being 'friends' vs letting her go), I truly believe letting go is best if you cant start competing on a new level. Before you reject my suggestion, please consider it at least briefly. If you don't see any strong, real reasons to support this seemingly drastic action, I can gladly(and sadly) elaborate on the why being friends in this situation is worse than losing her.
Until that critical moment of truth arrives, I might be able to think of some things that could help you change up the dynamic, if you're interested.
As a disclaimer, I'm not sure how old you two are, and age changes this dynamic slightly beyond the 30s, but you still pretty much fit the mold of a "nice guy," and the pattern still occurs even in middle ages. I know your pain, and I am truly sorry.