I think there are a couple of different things here.
The first is when to reveal personal info about yourself, right? For you, that question is a loaded one, and isn't just as simple as "my last relationship ended a year or so ago because we realized we wanted different things in life and parted on friendly terms".
For you, it's about past trauma, and revealing that to someone you barely know might not be a great idea, for your own protection.
So you can just say it's been a long time - which is the truth - or say something like, "you know, that's a really long story that I'd rather save for another time, but I assure you it's been long enough that I'm emotionally available for a new one", or whatever words feel comfortable to you. Then when you get to know them better, you can tell them more.
It might seem evasive or dishonest, but unless you want to do the life story conversation, that's what I'd go with.
The second issue here is how unworthy you feel. Everyone is worthy. Have you ever gotten counseling? Even if you have, you can get more. You have had a lifetime's worth of trauma, and you deserve to find peace. :)
I second Auntiejessie in everything she said. Also, you aren't going to get a lot of interrogation about what your last relationship was from the average person. I've never been asked this question in my life. So it's not like you will have to fend off ignorant but well-meaning people asking this all the time.
I hope you do have a good counselor who can help you unpack this. I firmly believe even the worst of life damage can be eased if you have a good therapist or counselor to talk to. In my most stressful time in my life I think I could have gutsed it through alone, but the counselor I found made it all so much less lonely and helped guide my thoughts in practical and more upbeat directions.