I am so sorry that you lost your DH at such a young age. Your mother and friends have said some very hurtful things. Who knows how any of us would behave in those circumstances?
I can’t imagine how I’d cope if I lost my DH. Sadly, I lost my own DF when I was in my 20s, so I can see things from your DC’s point of view too.
Have you spoken to your eldest daughter about how you behaved at the time? Going forward, I think it’s important to address what happened and not gloss over it. You sound like a very caring mum, so you may well have apologised already and mended this bridge.
When my Dad died suddenly, it was devastating. My DM completely changed. She did as she pleased and thought only of herself. It felt like I didn’t just lose my Dad, but I lost my Mum too. Overnight I lost the security of having a family home and parents. It affected me deeply. I felt completely lost and alone. I also had the burden of an unpredictable mother who acted more like a child.
She met someone else two years later. Despite me telling her that although I was happy for her, I didn’t want to meet him, she turned up with him unannounced. She’s also take him to family gatherings as her plus one without prior warning. It was upsetting for everyone to see them together because we couldn’t help but think that it should be my Dad standing there, not him. Also, he might have been one of many (he wasn’t - they’re still together) and I certainly didn’t want to meet every man she dated.
They’re still together and everyone has completely accepted them as couple. She deserves to be happy, and so do you.
However, my relationship with her has never been the same due to her selfish behaviour.
I don’t think you’re selfish at all OP, otherwise you wouldn’t be posting here for advice. Tread carefully though, especially where it involves your DC.