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There is probably an element of guilt there. Not because it's truth or real but because you want to please him. I would be concerned what else he is hiding too. He may have sexual behaviors that put you at risk or would break your heart. You sound like a nice lady and you truly love him, but that doesn't mean it is healthy to put your head in the sand. A relationship built on secrecy is worse than any relationship at all. Find your courage to move on. Your heart won't be open to a new healthier relationship if you stay with him. You will never know what could have been healthy if you participate in this charade that he has created
You shouldn't trust him is the answer. You will trust another man eventually - you may not trust this man. That is not the same. If he wants to wear women's underwear and you are okay with it, that is not a big deal. It doesn't harm anybody. If it hurts your sex life, then that is a problem but it can be worked out with a marriage counselor. What is more troubling is this relationship with this woman. If he is meeting up with her, eating lunch, and carrying on a relationship over the phone or computer, than that is inappropriate, hurtful, and down the line going to be sexual. People don't get past that. You aren't unusual. They would feel exactly like you do now! They would have no trust, they would be hurt and ready to separate, and they would be offended. If it isn't sexual now it will be. Lunches and messages lead to other things. You have to find your strength and decide where this relationship is going. If you are parting ways he has to go. He can't have his cake and eat it too. Let him go before he hurts you any further.
Masturbation can be hurtful because you probably are feeling that he prefers that activity. This doesn't necessarily mean that that is the case. Sometimes masturbation is pleasurable because the sensations are different, but it doesn't make you less attractive or desirable. You are on the right track with counseling both his and yours. Try to relax and think about what you would like to address. you may even make a cheat sheet before your appointment. Your moods are inconsistent because you are going through this turmoil. Just a thought but you may consider an evaluation for meds due to situational depression. It's okay to ask for what you need. You do have a lot at risk so that is an even bigger motivation to give this 100%. You are free to make your decisions based on what you both wish to happen. He may want the divorce today but he may or may not want it tomorrow. Stay in therapy and deal with these issues with a professional. Best wishes.