Letting my guard down ( advice? )

I’m having a very hard time letting my guard down. I’ve been talking to a guy for a year now and it’s slowly going somewhere but every time he tries to get close I shut down. It’s like I’m sabotaging my own happiness because of the previous relationships that I’ve been in. I feel like I can’t trust anyone. Because every time I do something bad happens and I end up broken hearted and even more guarded than I was before. But the problem is I really really like this guy and I feel like he might actually be “the one” but I can’t seem to break through this wall.

Any advice?