The hardest part is to try and stop the what if's could've and should've thinking. It's hard to sit with that and not have feelings of shame and regret. When you see your therapist, I'm sure they will advise certain techniques to help you stay in the present. There's no changing the past, or seeing how you could do differently in the future without driving yourself nutso. This was a struggle for me, but with mindful practice, time and patience... you'll get there. Hang in there- you can't fix the past nor predict the future. Work with the present in which you have control of. Best wishes 
Regret. Personally, I try very hard to stay in the present. If I'm in the past, or projecting into the future, I'm being robbed of "the gift of this moment." In the beginning of my recovery I was constantly in regret. Living in the present comes with practice and the passage of time.
The serenity prayer, incense, essential oils, a hot shower (love them) a walk in the park. Maybe call someone empathetic to center myself
Regret was the cause of many of my relapses. When I finally focused on boundaries I made I through. Truth is you may have to NOT suck it up for everybody you ever wronged and you may have to tell some people "yeah I was wrong and you're not getting over it and that's why I will no longer be in your life. "
Could ofs and should ofs were a significant reason I kept drinking. Drinking away my time resulted in my drinking to calm the feelings of regret and failure. I simply had to work on acceptance. It took time, and I still find myself wallowing on the past, but I have accepted that I should have, but didn't. I have to look forward and live in today to secure my future, one day at a time.