You say for the most part you aren’t unhappy but you don’t say that you are happy either. You’ve used a lot of double negatives to describe your relationship.
You need to work out what you want from your life and relationship. We only get one life and it seems an awful shame to spend it with someone you ‘don’t not get along’ with and who doesn’t actually make you happy. Sometimes relationships come to a natural end, this does not make the relationship a failure.
If you compare it to getting your dream job, you put in time and effort, passion and loyalty. You spend years developing your skills and being an excellent employee. Over time issues arise, the job is less enjoyable but you stick at it because you’ve invested so much. Ultimately there may come a time when your needs change, or the company changes and you no longer feel the same connection to the job that once was so important to you. Leaving that job wouldn’t be seen as a failure, everything you accomplished during that time still stands but it’s time to make a change. A new adventure for you that will be fulfilling again and the opportunity for the company to grow with a new employee who is excited to be there.
Obviously ending a relationship is a big decision and not an easy one to make but it doesn’t sound like either of you are happy with how things are.
Staying together, if nothing changes, means agreeing to a lifetime of celibacy and no intimacy. You say you don’t feel like partners so other than not living alone, fear of detangling your lives and a shared history what are you actually getting from this relationship? Is there financial security, companionship, shared interests? Even if these things do exist, are they enough?
By staying together you are depriving each other of the opportunity to be happy, either on your own or with someone new. Do you know that your husband is content to live his life as your flatmate? Maybe he is just as uncertain as you about your future but is also afraid to address it.
Dont fall into the sunk cost fallacy - there is nothing worse than staying with the wrong person for 30years than staying with the same person for 30years plus 1 day. Separation does not negate the years of history and children you share but staying together as you are will cause resentment and bitterness to taint the times that were happy.
How long ago did you have counselling? Would it be worth trying that again? If not then maybe individual counselling to help you work out what it is you want and help him deal with his anger. Couples counselling can also be useful for working towards amicable separation.
Ultimately you need to speak to your husband. Explain how you feel and ask him to take some time to consider what he wants going forward. Apologise for what happened if appropriate but remind him that it is unfair for him to hold this against you indefinitely. You can’t change the past so he either accepts that, forgives you and you move forward together to rebuild your relationship or he can’t and you decide whether that’s something you can live with. If not then you have no choice but to separate. If you are concerned about how to say this or how receptive he will be you could write him a letter or address it in a therapy session.
You both deserve happiness and shouldn’t be forcing yourselves to stay together because the alternative is too hard.