How to trust?

I had a bf who cheated on me with my best friend and other girls. I also dated a girl who ditched me for another guy. I have a new bf now and he's so kind and he's different from them. How do I learn to trust in a new relationship? It's just a bit painful to be vulnerable again... We've talked about it but I just can't seem to? How long did it take you to trust someone and what are some good signs you saw in your significant other?