18 is very young. There's no reason for you to rush into such a big commitement. If you continue to have such doubts, then you definitely shouldn't be considering a marriage with this person.
Take your time and work on either strengthening your relationship with this person or find someone else. Either way, marriage should not be on your mind right now.
However, if he shows signs of being abusive and controlling, then it's definitely not recommended and it's better for you to get out.
