Good evening. First allow me to say that I am truly sorry to hear of the difficulty and ambiguity you are facing right now. I have read through your question thoroughly...twice in fact to ensure that I read everything clearly.
Based on what you have identified the relationship you have been has been fraught with turmoil, pain, and infidelity throughout it's entire course. It's been truly unhealthy from the start in that you both have consistently worked toward harming each other and then reconciling. The relationship has not been built on trust and honesty and to be perfectly honest with you I see it only growing worse if you stay together and even if you marry one another. Please forgive my abrupt manner in the way in which I have answered your question but I feel this is what you need to hear in order for it to get through.
Please feel free to ask me any questions that you may have or if you need further clarification on any of the suggestions I've offered.
If you feel I've answered your question to your satisfaction I would greatly appreciate your clicking the ACCEPT button thereby giving me credit for the answer and suggestions I've offered you. Thank you for the opportunity to help and I look forward to your response.