How to trust my partner again?
You are so strong for reaching out and asking this question. First have you guys tried couples counseling? That could be very beneficial for you guys during this time to try and work through this and talk it out with a non judgmental, unbiased person that does not know all the other life details that you guys have going on.
Nobody has to understand the choices that you decided in the relationship. This is ultimately between you and your partner and nobody else. You are completely right people make mistakes and nobody is perfect. And you do not have to explain that to anybody else.
Building trust in your relationship can take time and patience. It would be time for you guys to sit down and have a deep conversation about these intense feelings you may have felt. This is not the time to push away the feelings or ignore what happened. Finding a way to be attached emotionally is going to be important in this process. Do you guys spend time together everyday? Finding something that you can do, just you two, no phones and really getting that quality time in. Get to know each other again on a different level.
Are you focusing on the present and the future? Try not to let your mind wander to the past and the past mistakes. What can you and your partner do to be more in the present. What are your expectations for the future and what are his? Decide together as a team, partnership how to move forward together. It will take work from both of you.
It is also important moving forward that the communication is talked about and how it can get better. You need to be open and upfront with your partner about your feelings and the thoughts that you are having so you are not keeping them bottled up inside. If you are worried and concerned then you need to tell him because if you do not then you will turn into a helicopter partner always checking on him and seeing where he is at. When you are talking about these feelings it is important to speak with "I" statements because that shows you are taking ownership of your feelings and thoughts.
Also remember to be kind and nice to yourself. You didn't do anything wrong. You are growing and learning and it will take time