Need to get this out...

I have been married for 15 years. All of which I have been heavy. Heavier now than I was when we first got married. I am not proud of it, I have tried dieting and working out and those things seem to work short term. I end up getting disappointed in the lack of change and give up. Even though I know changes aren't always visible on the scale. It is a battle that I go through every time I set out to lose weight. Well recently my husband has told me that he loves everything about me but my weight and it is just not something he can get over. He has tried helping me in the past, we usually just end up fighting because he tries to get me to do things that are beyond my abilities. Where we are now though, is I either fix it or we go our separate ways. He isn't expecting changes overnight thank god. I love this man and the thought of not having him in my life is devastating. We have been fighting a lot lately and it has a lot to do with my weight. Here is the other thing...I was going through our cell phone bill and noticed a lot of texts between him and a number that I don't recognize. They started off slow but then got to the point where it was going back and forth all day. So I asked him whose number it was. Apparently it is an intern at his job. A female intern. He was training her and they became friends. He then said that he started to develop feelings and she was as well. Someone at his work saw this girl hanging around a lot and asked him what he was doing. He said it was then that he realized that he had let things go too far and he put an end to it. Which was evident by the phone records. The texts to and from that number stopped abruptly after going on for about 2 weeks. We discussed it, I understand it. He said it was never anything more than texting. No touching, no secret meetings, no pictures, just texting. I asked if I were to read the texts would I be hurt, he said no because it was pretty much them just getting to know each other. I trust this man. He didn't try to lie about it, he came clean when asked and then answered all my questions. He said he didn't want to say anything to me because the interns last day was today (which he took off so she wouldn't come see him). He also said that he didn't say anything because he realized he was wrong, he was ashamed and he didn't want me to feel defeated and stop doing my workouts because I have been doing so well and have actually started seeing results. I want to be mad but at the same time I am having a hard time doing that because 1) he opened up about it when asked 2) he is honest about his feelings 3) because even though he allowed it to start he put a stop to it. He said that he sat back and he thought about things and said it wasn't worth giving up what he has. Sorry this has been so long. I know everyone will have different opinions and all are welcome.
Update: thank you for the kind responses. I do feel like he is a good guy. I am extremely grateful that he made the decision to end his new friendship/potential disaster. We have a 10 month old child together and our family was important to him. I made sure to let him know That I appreciated his honesty and his decision. As for the weight loss, when I started out several years ago it was for him. Since having a baby I know now more than ever I need to do it for me. I need to be able to keep up with my kid and he deserves to have me around for years to come. As far as surgery goes, it is not for me. I have researched it and I have witnessed first hand the affects it has on the body years down the road. I know I can lose weight, I have seen some results, I just have to keep the motivation...and my family staying together is a huge motivator.