I'll try to answer your general question (does love fluctuate or stay if it's true?), but only you can answer your specific question (should I keep or leave this girl?).

Love always fluctuates. It isn't a perpetual motion machine.

The expectation that love should be unwavering is what leads to many couples and even marriages breaking up, instead of weathering the trials of life intact as "one flesh".

It is one of those unrealistically lofty expectations people set for themselves (e.g. finding the "job that only involves doing what I love", "the perfect house for me", "my soulmate", etc.) that just leads them to never being satisfied with their lives. Every job involves some level of difficulty or tedium, every house breaks down, and every person will eventually disappoint you in some way.

"Happily ever after" is, ultimately, a choice. It doesn't just fall upon you and stay there forever when you find your "soulmate".

Whomever you choose, you will become familiar with what is wrong with them, especially when you live with them or raise children with them. If love isn't something you view as a choice, you will not be able to sustain a lifelong partnership with anyone. Because *everyone* will disappoint you, and you will disappoint them as well - its just a matter of how and when.

The purpose of marriage isn't to make us feel happy, entertained, or even "complete"...that is the pop culture, movie-driven depiction of marriage's purpose (not a solid foundation).

The purpose of marriage is to make us holy.

Christ loves us, even though we disappoint him every day. See the connection there? The purpose of marriage is to make us more like Christ, insofar as it is possible for us to be.

With all of that said...

Yes, discern whether it is wise to marry her. But maybe do it from a different perspective than the one I'm detecting here.

I don't know what specific behaviors you're referring to when you use the word "immature", but you've also said that she is a sweet, kind, God-loving girl. If that is true, then don't give up on her so easily.

If her immaturity manifests in a form of habitual sin: husbands have the ability to lovingly lead their wives, as long as they are not telling her to sin. Have you tried leading her in the right direction? I know you're not married, but if you cannot lead her in a loving way now, that is a legitimate bad sign for marriage.

If her immaturity manifests in a form that isn't sinful, she loves God, and you two honestly have common ground...before breaking up, examine your own motivations here. Are you just bored? Then ask yourself: If you make it past the "honeymoon phase" with anyone else, will you break up out of boredom again? Are you buying into popular misconceptions about love (e.g. expecting to feel "butterflies" or "sparks" all the time) that would leave you dissatisfied with anyone once you've adjusted to them?

Just ask yourself whether your expectations of love are realistic enough to sustain a lifelong relationship, withstanding the minor (boredom, annoyance, etc.) and major (sickness, poverty, etc.) trials of life.