Hi, luna, welcome to SI, the best club no one wants to be a member of. So sorry you find yourself here, there is a great deal of support and knowledge from members who have walked in your shoes.
Your husband's cheating has absolutely nothing to do with you. He owns his actions.
Gently, your husband is a serial cheater, at least for the past 15 years and at least 6 times that you know of.
It takes years to forgive, are you certain you are willing to forgive over a decade of cheating? I'd put him in the club of someone who is most likely to cheat again.
IMO your husband should not be having any contact with his affair partner, he made a mess of this, he needs to actively seek a new position pronto. My husband had an affair with a co-worker who lived 3,000 miles across the country, there was no way in hell I'd allow any more communication or ever allow him to visit her site again. He found another job, no compromises.
Please find a good counselor for yourself to help you navigate this hell your husband brought into your life. Also check with your MD for STD testing and temporary medications to help you cope, even just a sleep aid to get you through these next few months. Your husband dropped a nuclear bomb on your life, the fallout is wide and it will take years to move past this nightmare.
Understand that most serial cheaters don't change. How can you be absolutely certain their affair has stopped? Many affairs go underground, burner phones, secret emails, snapchat.
I cannot trust him 100% anymore. I am afraid he will start another relationship when he feels low. Also he keeps telling me that he learned his lesson involving himself with co-workers now but should I believe him?
^^^Right now you cannot trust anything he says. He's been leading a double life for over a decade and all of a sudden he is sorry? He learned his lesson about affairs with co-workers , what about the business trips and dating site. Your husband was/is actively seeking other women outside of your marriage. He is a very broken person and there's not a thing you can do to fix him.
Please take time to read the articles in the Healing Library.
I'm sorry if I cannot be more positive in my response, but your husband has lied for 15? years, and now that he got caught, he's willing to change. What is he doing to help you feel safe? Counseling, perhaps a sex addiction specialist, has he given you access to phone, emails, social media, voicemails, etc? Accountability for his whereabouts?
BTW is the co-worker married? If so, her husband should be informed by you that he's living a lie. Whatever you do, don't tell your husband if you choose to do this, and do not tell him about this site.
Sending a virtual hug, we all understand the emotional roller coaster you have just boarded.