I need serious advice I just found out

I just found out my fiance of nearly 8 years 4 years engaged and 4 years dating has been cheating on me for atlest the last month and have no idea what to do I already confronted her when I found out this morning I know it’s most likely my fault that I drove her to do so with my actions / lack there of I’m not the best person for communication and I have a lot of issues that for the past few years I could figure out how to handle with out medical intervention like my adha my major laps in memorie on conversations we’ve had but only recently I have decided to get medical advice and I have an appointment for a physical and will be asking to get a full psych evaluation this week I want to know how can I fix this relationship I love her more then I do my self and she doesn’t believe that I have been trying to better myself to get us back on the same page we’ve been arguing more and more lately over the smallest things iv been trying to fix on my own I would love to give you my back ground on how I grew up but I can’t remember most of my childhood most of the memories I can’t remember is after I graduated high school all I remember growing up is I was mostly a loner had 4 friends I would hang out regularly nut have lost contact over the last 15 years or so until I met her back in 2014 we started out as friend and started un officially dating in 2018 officially started dating in 2019 got engaged in 2022 iv been trying for the last 2or 3 years trying to work on how I can better my self with out asking her how I can do it and a few months back we lost our house and were in Airbnbs for 2 months until me found a place in late may and since we moved into the new place it was good for most of the first month there and now it’s constantly arguing and now that I’m making the effort to find out what’s wrong with me I find out she’s been sleeping around with another guy and how I found out and I’m not proud of it but I went on her iPad and found the messages I want to work things out with her and get the woman I fell in love with back and don’t know how and I’m asking complete strangers for advice and I need help figuring this out