Love and doubt can exist in the same relationship.
That does not make you disloyal.
Marriage is a large decision. Some uncertainty is normal.
The question is what the doubt is made of.
"Marriage is scary" is different from "I do not trust this person."
"I am nervous about change" is different from "We want different lives."
Do not put every doubt in the same category.
Normal fear is about the size of the step
You may think:
"What if we change?"