Hey I'm 19 and in my first serious relationship and I need advice.. I'm feeling so insecure and like I'm just pushing him away please help!!

I've never dated before.. Only had hook ups, no feelings involved.. And now I'm dating this guy and we're saying I love you's and talking about marriage.. But I'm fucking it all up by being insecure.. I can't believe his complements and find it hard to believe he's faithful even tho I've got no reason to think otherwise.. I've always struggled with self esteem issues but I've never wanted to open up about them so much and I know I'm being a crazy insecure girlfriend.. I don't want to lose him what do I do?!?