I mean maybe you aren't really ready for a relationship. Relationship is literally unknown, you don't know whether it will be long - term or short term. Putting a guard up is not healthy behaviour, it prevents you showing you the real "you" to the other person, and tbh that will be straining for you in the long run. I feel like everyone goes through this, I mean one day that might happen, but you just have to pick yourself up and move on. There will be other guys, this guy won't be the last guy you will ever get to know. You have all life to meet amazing and not so amazing people.
So, if you are ready to be in a relationship, you have to understand it is important to communicate how you are feeling, and keeping a guard up, will only give the other person mixed feelings.