Ah yes.... the loser magnet. I had (?have) one of those too! Mind you I am now (thanks to Baggage Reclaim) much more aware of how I have previously ignored "red flags"; if i'd taken heed of them with my ex I would have saved myself 3 years of hassle!
Ah Claire hon.... accidently? What ... .did he slip and fall and just happen to land in her? He chose to sleep with her meaning while he is no mass murderer make no excuses to yourself about this aspect of his behaviour. His behaviour in this case is about him; your responsibility is in why you are not holding him fully accountable for it (yes you may have barred him from your home but you are clearly very still emotionally involved with him) xx