@dee1015,
She does have a point, skewed by her past experiences though it may be.
One of the scenarios we see here a lot are people who get together early in their lives and then, years later, feel they missed out. This doesn't mean that everyone feels this way, or it's right or wrong, etc. But there is something to be said about having other experiences.
Beyond that, though, she is of course projecting her own experiences and insecurities onto you. You say she has been in therapy, but it doesn't seem to have stuck much. This just seems like her own personal anxieties. Note, I am neither a doctor, nor am I a therapist.
Ultimately, of course, this is your life. You are an adult, I assume your boyfriend is, and so you are both free to choose your lives. As one of our members here says,
know which hill you want to die on. That is, pick your battles.
I would say that this is a big battle. I would say that this is a good hill to die on. But be prepared for the relationship with your mother to potentially die on this hill.
You can acknowledge her feelings and her concerns, and address what I mentioned above. Explain that you don't feel the need to sow some wild oats. You're happy and this is important to you. You're not going to go and deliberately sabotage this.
Say this when you are both calm, just sitting at the kitchen table.
And then gauge her reaction. She might respect you standing up for yourself. She might whine but ultimately be okay with it. She might try to passive-aggressively guilt you into doing what she wants. She might scream at you that you're making some horrible mistake. She might claim you are no longer her child. Any of those things could happen, or other things could.
But I think you need to have the discussion at some point. Be prepared for the worst, and recognize that you might not be able to have your love relationship with your boyfriend and a semi-normalized (it's not a normalized one by any means) relationship with your mother at the same time. But this is neither your fault, nor is it your boyfriend's. It's your mother's.