Hi Maria,
You may have emotional defenses up due to a past relationship that failed.
To be okay with being vulnerable, you need to look at this guy as someone completely new and not connected to anyone that you have dated in the past.
A tangible way to do this is to avoid going to the same places or engaging in the exact same activities with this guy as you did with your ex or exes.
This helps to keep you from being constantly reminded of your past exes and those failed relationships.
Instead, be open to trying new activities and adventures with him.
This will help to create a distance between him and your exes, both in activity and personality.
As time goes on, you will start learning to let go of those defenses and trust this guy.
That is how you will ultimately get to the point where you will be okay with being vulnerable.