Women, Men, Anyone! Please help!

Hey I am 22/f and my boyfriend is 23/m. We have been together now for 3 years and it is starting to go down hill. We had the best fairy tale relationship! I was so happy and it seemed so was he. We were taking vacations together, sex all the time, we were very open with each other and we never fought! About a year ago I found him talking and sending pictures back and forth to another girl very inappropriately. Through a lot of talking and compromises I decided to give him another chance however, since then I can not find myself forgiving him for that and my trust for him is gone. The sad thing is, that I cant even find myself wanting to be physical with him anymore. So, I decided that we needed to take a break so I can try to figure things out and we have been on one for about 3 months now. Here is the thing now that I need help with. For Christmas, he bought me a beautiful ring and he told me that all he wants is to be with me and he is so sorry that he messed everything up. He said that this ring is a promise that he will never hurt me again. So now what do I do? Just because he gave me a ring and promised to never do it again doesn't make the trust come back and I still cant find myself letting go to get physically attached to him anymore. I don't want to be unfair to him and string him along in this thing that we are trying to hold on to. I DO love him but its hard for me to get back in that fairy tale relationship that we had. Does anyone have any experience or stories similar where there relationship worked out in the end? Is there any advice or input on the situation? Thank you for taking the time to read.