I am a person who used to trust way to easy and got burned a lot as well.
My therapist told me the following.
He said:
You are like a house that has all doors and windows wide open.
Even the gate at drive way is wide open.
People storm through the gate into your front door,
go crazy in your house and leave through what ever door.
Your sitting there and wondering, what did just happen ?

Then he explained to me how he handles this in his life.
He said:
When somebody arrives at my gates, I will go out and see who it is.
Talk to him through the gate and get to know him.
If after a while he seems OK I will let him through the gates.
Then at the front door I will taken an even longer time to get to know him better.
When that seems Ok he can come into my house.
Even then I will keep an eye on him/her.

I will never forget what he said and force myself to protect myself better.
Its hard since I am an open person by nature, but I am learning it.
It does not mean I will never get hurt again by trusting but I will do my best to avoid it.

I am sorry to disagree about what you say that being betrayed makes you stronger.
If it happens to often it can brake you down and maybe to a point you choose to not let anyone come close anymore.
It all depends on the persons character :)
Otherwise good post :)