lost someone special

Ive been dating somone close for 2 years and it appears she may of moved on and im not sure what to do. Our relationship started off decietful, my complete fault. Just thinking about it is hurting me and wish i was honest. When i first met her, i was living at home with my family. I was going through serious problems at home with my girlfriend at the time and decided to look outside. I was living with now my exgirl and daughter. So i met stacy while i was living with my family went on a few dates and was not upfront with her until 3 months later and this was after we had sex and all her feelings were involved. She was hurt,upset. After, we talked about it she decided she wanted to continue the relationship. So we continued the relationship and became really close emotionally and spirituality. We went on 3 trips together, read novels together. It was great During this the my relationship at home started to disintegrate. Then it finally ended. So fast foward until now, when my family left, i never commited to stacy and i treated my realtionship as like it first started. No progression, no growth, just casual dating. At the time i just wasnt ready to quickly move foward and give her what she wants marriage and kids especially i was coming out of something. So stacy would complain she feels like "the other woman". So one day i suddently one day i noticed the phone calls stopped coming. I called her few times and after a few days she said she thought about it me and her wanted different things. It really burned cuz the few days i didnt speak to her i was lost. After i spoke to family and friends, they basically said i should of really made some form of commitment with her but i never thought she would just pulle away. So i thought it about it for days and told stacy i want a serious commitment and will like with marriage with her. I completely caught her off gaurd and she didnt expect it. She said she needed time to think about this. So we went out a few times afterwork the chemisty felt the same. Once i saw her i absolutely was comfortable with committing to her and wish i didnt wait to long. we talked about marriage, kids and the future. During this time i told her i was having a bbq for her so she can finally meet my family. The strange thing is Stacy had disappeared again, no phone calls and no response to my text. It went on for one week and then she finally responded the day of my bbq. She explained how she needed time to think about why all of sudden i was coming foward, when she wanted this all along. So moving foward, she comes to my bbq at almost 11pm which she knew was for her. And stays for one hour. Im pissed...im asking myself whats going, because im thinking we are working on really working being commited to one another. So the next day we had plans to see each other and again it became antoher dissapearing act. So i home laying on my couch and thought heavily. I decided to get in my car and pop up at her house. So i did and when i got there it was about 11. I knocked on the door and she opened it we both stared at each other. I already knew. There was another man inside the house. She had on the same shorts me and her used to go to sleep with. She got dressed came out to talk to me and told shes been seeing someone else for the past weeks. I asked her why didnt she tell me she said because she loved me and didnt want to lose me. Now mind you, we had sex the night before at my bbq in my room. And i sure she slept with this guy too maybe that night. So next day, she called to explain everything. She said basically shes been wih me for 2 years and shes been the other woman. She met someone else so why should she throw that away, not knowing what she would get from me. She said shes not even sure about the guy. Then she said she loves me and didnt want to let that go. She ended it by saying shes stepping back from the situation from me and him to have time to herself. Its been a week and im unsure what to do. Some days im low, angry, and bitter. I feel regretful i didnt take this relationship more seriously. It not a minute im not thinking about this woman and the situation. We had really good time and i remeinisce a lot. I really miss her and unsure what to do. Some friends say buy her a ring now and go to therapy with her. Others say the damage is done and move on. Im unsure what to do.