We, all more or less, feel guilty for taking part in our M breakdown. We are only human and at times we do things to protect ourselves. As my IC it takes two people to make a M works, so don't be so hard on yourself. We all make mistakes but we can learn from them. In our case I think we feel down because we think about what we thought we had, and what we could have done differently. Unfortunately we can't undo the past. We need to live in the present and enjoy each day. I know easier said than done. What do you like to do? Do you do any sports? Have you got any friends to go out with?
As my IC told me this is huge betrayal we are facing, and she added that we need to put the focus back on us. We cannot control what WAS does, only what our life. I humbly relate to how you feel, could it be we are hurt because we feel worthless as our spouse chose to go with someone else than us. My IC says it does tell a lot about them and their values than about us. I hope you'll have a better day today.