Is my hubby's affair over or is he cheating on me?

my husband had an affair 6 years ago.....the female got married and they were not in touch.........she came back a year ago and they started again....i found out they were exchanging mails....when i asked him to choose between her and me he said me.........he says he is trying to forget her. if she had loved him she wouldn't have chosen another man who was doing much better.....when she joined back office a year ago she found my hubby was at a much better position and status they started again. he now tells me he is not in touch with her how can i believe they work in tme same office. need help. i am married to him for 12 years we have a son.....we also had an office romance.....he was very possesive about me he cant stand anyone even keeping a hand on my shoulder......was he comfortable with her getting cosy with her hubby...her hubby is no more.