LOL Of course she says she isn't using you! Who would step up and admit "yes, sir I am in fact using you, once I am done you will be gone." No one, that's who.
You walked right into this one... you are indeed the nice guy that will finish last. You are in love with a woman who is using you and your money to benefit her children. You claim you are in love, ok fine. But you do realize she does not love you back right? If you would like to remain "stuck" then fine accept the situation as is, becasue clearly she has you submissive and trapped.
Advice for you is useless you refuse to do anything about it.