Sweet makes a good point. While there are some people who at 18 can be very grown up and responsible it is rare. The OP doesn't, I don't think, state his age. He sounds young too. The relationship is 4months old - the people in it are young - it doesn't seem to be working, they seem to be wanting different things and have different ideas of what is OK and what is not.
I think the OP is better off cutting his losses and moving on. From experience the crying and 'you've changed me' speech is likely to be BS. I make no value judgement on her having several partners by age 18 - if that what she enjoys she should go for it (remembering to be careful) but she shouldn't try to be in a committed relationship because it doesn't sound like she's ready for it. Therefore the OP trying to stick with it and 'just trust her' seems somewhat pointless. They want different things.
I think the OP is better off cutting his losses and moving on. From experience the crying and 'you've changed me' speech is likely to be BS. I make no value judgement on her having several partners by age 18 - if that what she enjoys she should go for it (remembering to be careful) but she shouldn't try to be in a committed relationship because it doesn't sound like she's ready for it. Therefore the OP trying to stick with it and 'just trust her' seems somewhat pointless. They want different things.