Why betrayal is so hard ?

Betrayel/infedellity can give you a chock/trauma in your nerveussystem and it can push you further into a more insecure attatchment style. It's very understandable from a nerveussystem perspective.

"A trauma is any event that breaks through the body's stimulus barrier and leads to overwhelming feelings of helplessness. Freud
We are being traumatized if our ability to respond to a single threat is overwhelmed in one way or another (too much, too fast, too early especially if we can't reach successful resolution.' (Peter Levine)"

Some people work with this in SE/chock trauma/ attatchment therapy.

Just because one person betrayed you dosent mean that all future partners will betray you.

Have you tried to work with this in therapy ?

I have posted something about infedellity in the healing desorganised thread and the "how to create a long lasting relationship" thread, "healing a broken heart threa d" ect - general discussion forum.

Sounds like infedellity will be on your list of deal-breakers in the future.

Just because that the betrayel happend in your lovelife dosent mean it will "fuck up" the rest of your life.
To me it sounds like you are protecting yourself from getting hurt in the future.
The "casual love relationship" with the guy you are seeing can be okay for now.
It can be okay for a while, but it can also be like "dating" safely, like driving a car with the speeder and brakes pressed down at the same time and like always having a backdoor open..

Isn't being too afraid to engage in full intimacy, though it was something you wanted before, letting your ex eff up your whole life?

A lot of people don't cheat. They try to talk and work through issues, and end relationships before moving on. Cheaters often have bad communication skills.

Never fighting also is usually a sign of bad communication. You may fight more now because communication was blown open by the betrayal whereas before he was acting happy and not complaining to anyone, but in reality was shoving his feelings down until he couldn't stuff them anymore and acted out instead.

I think the difficulty in feeling how you are at this point is not because you don't trust future partners, it's because you don't trust yourself (to know how to choose a partner who won't betray you) or your own boundaries (you're clamping down totally rather than risking letting someone in in a healthy way). This is understandable -- you were blindsided, you were hurt, you were betrayed, now you'll question everything you thought you knew about your romantic relationships and partners.

Though you need time to heal and mourn, shutting yourself down isn't resilient or moving forward. It is hurting yourself more out of fear. That is a typical problem for insecure attachment types, especially FA. You may not feel you know what else to do, you did your best before and still got hurt, so what now? If you don't trust others or yourself, then you simply shut down or try to convince yourself you can control your environment (control yourself and others by choosing relationships without real commitment, even if that's what you really want as you enjoyed being married until you were betrayed). Insecure attachers want to feel in control of themselves and their surroundings, even when it's not things they actually can control. But it's an attempt to combat feeling powerless and fearful.

To combat this, it isn't about gaining more control. It's about strengthening your own sense of identity so that if you get betrayed, you don't abandon yourself. Based on what you said happened earlier in your life, a good therapist probably can help you untangle some of this and give you better tools so you don't feel powerless and distrustful of others and of your own ability to choose good partners. That's what you're asking, what can be done about this ie what are better tools? That's one of the things therapists are supposed to be able to provide.

What tools are you looking for, then? If you feel that you're content with your situation, what work will you be motivated to do to change?

Betrayal is a violation and can be hard to process, but processing it is another form of going through 5 stages of grief.

Denial, Anger, Bargaining, Depression, and Acceptance.

You may be in bargaining right now (with yourself). That's normal and on track, you just don't want to get permanently stuck in any stage before the last one.

Honestly, it sounds like you adopted a different mindset based on the betrayal and are happy with everything in your life...so I am trying to figure out what you are trying to learn from others?  Because grief is a rather individual thing.

I think a therapist is the right approach.  My therapist has helped me tremendously.  I highly recommend an SE therapist because sometimes trauma gets locked inside the body and an SE therapist can help work that through.  I know of at least 1 DA on this board who found her therapist really helpful.  Good luck.