Relation advice?

So she took a liking to me after that we went out a few times and then started a relationship. Now this is my first relationship and i feel that its moving pretty fast, she agrees with that aswell, because the other day she told me that she was in Love with me and was surprised because we basically only have just started dating and it was too soon, which i agreed with aswell.

Anyways the relationship seems fine with me and we try to be 100% honest. Should i be worried or anything?

As in i know how we met so whats to say that she is faithful i always have that thought in the back of my mind. I also that she may be taking advantage of me being new to relationships and may just be saying things like i love you to basically take advantage of me.

Am i overthinking things? I told her most of this and she said I'm overthinking it but in my position i think most people would have the same thoughts. But i do care about her. Do i just go with the flow or what?

Also im 21 and she is 27 if that makes a difference. She always tells me she feels that i will leave her since its my first relationship and ill think there are better options out there.