How can I let my walls down and let someone in to show them I can trust them after being hurt?

It is completely normal to feel afraid of getting hurt since you were hurt in the past. Because you were hurt in the past, the mind registers the experience as an ongoing potential threat and therefore to be on high alert if there was potential for this to happen again. It’s as if the mind created an important file and because you still react to what happened in the past, the mind continues to register it as important and brings it to your attention that you need to look at it when actually it needs to be deleted if there is no use for it anymore. The mind projects possible scenarios into the future due to past experiences even if there is no evidence in order to try to protect you from harm. Also, neuroscientists explain that the negativity bias influences human behavior because evolutionarily humans needed to be able to recall negative information for potential threats as a survival promoting mechanism. Back in the day if a wild animal were to attack, it would be important to be able to recall this information in order to figure out a way to avoid it and survive. Humans don’t face the potential for an animal to attack as much but the threat is often perceived as interactions with others and a threat to one’s sense of self. Really it’s like we need an upgrade to recognize when we can let go of the past and when to relax into the present moment. Continue to come back to the present moment and realize that you are safe and connected with your partner if there is no evidence to consider otherwise. 

Since you really care about this person; as you continue to build trust, you can notice on a daily basis if you have any evidence to support the thoughts related to the fear of getting hurt. If thoughts arise that provokes fear: notice them without reacting to them. If you start to relate to these thoughts in a calm and accepting way they won’t register them as a need to continue to bring them to your attention as they will register that there is not a potential threat; it’s the reaction to the thoughts that keep them perpetuating. Spend time focusing and noticing all the things that are going right in your relationship to continue to nurture and move the relationship in the direction that you want.