married ending and Seeing someone else

Well I was married for 20yrs and just separating and hope to finalize separation by June. Its been hard to separate because we have kids. I haven't felt love in our marriage for a very long time. We were young adults when we got married because we got pregnant with our first child then much later we had the twins. This was my first and only real relationship. While I'm happy to be separating it's hard. I have been seeing someone else for almost 2 yrs now and this person knows my situation. We do have our ups and downs but I have felt things I've never felt before with this person. I'm just looking for advice on situations that are coming up. For example today I mentioned to the person I'm seeing that I received a june wedding invitation and it was addressed for my spouse and I. The person I'm seeing expressed that they wouldn't be comfortable if I went to the wedding with my spouse as friends. I didn't think it would be a big deal and this lead to a big argument. Am I in the wrong? what is the right thing to do?

you're either with someone or you're not. it's call monogamy. so, yes, this is a big deal. and, yes, you're wrong. and if you want to maintain your relationship with the person you're seeing, you won't go to this marriage with your spouse. you can go separately but just not together. it would give completely the wrong impression.

Dealing with friends and family will be awkward during this time. People might be inclined to choose 'sides'.
I don't think that it is really about who's right or wrong, it just is going to unfold this way. Follow your heart, try to respect other people's feelings and understand where they are coming from when they disagree.
Explain what is happening and what your intentions are to your kids. They will especially need your support during this time.
There will be losses, gains and compromises. It will be a big deal to some people.
Best of luck to you.