Reassurance is different from validation and normalization.
Anxious kids - over about age 3- who get reassured have their anxiety fed because it reinforces the anxiety (counter intuitive but think about it logically. If I say "I am worried about X, Mum" and you reply "Don't worry, I will be there for you, I will help you with X" then you are quietly reinforcing that X is a very scary thing that your child has a good reason to fear, and that they can only cope with your input, and indeed could only be expected to cope with your input).
Anxious kids who get dismissed ("suck it up, buttercup" or "welcome to the real world" type responses, aka Dad) have their anxiety reinforced because they are shamed and feel unsupported and their anxiety is not contained.
The best thing to do is to take the anxiety, validate that feeling, normalize it, and assure the child that THEY can cope.
"Mum, I am worried about X"
"Well, that's not surprising. You've never done X before. Lots of people feel worried about doing new things. You are bound to feel a bit scared but I can help if you want. Shall we look again at what you are scheduled to do?"
In this case "Mum, I don't like Malcolm much. I feel like he's coming in between us"
Option 1: reassuring - feeds that core anxiety
"Oh darling, don't worry, you are my number one and nothing and nobody will ever come between us. I will always be here for you and I will never let any boyfriend change that". (Note, over reassuring often doesn't really tackle the stated problem either, or give any actual strategies).
Option 2: dismissing - child feels unheard, unsupported "I am a grown up and I get to have a life too. Malcolm makes me happy, so suck it up, buttercup".
Option 3: validate, normalize, and explore:
"Well, thanks for telling me. You know what, it's really not surprising that me having a boyfriend feels a bit scary to you, after all, since you were a tiny baby it's just been me and you. It's bound to feel a bit weird, isn't it. Lots of kids find it weird when their mum starts dating people.
First off, you know I love you VERY much and your welfare is my top priority. And Malcolm understands how hard it must be for you to get a new adult in your life. Let's talk about how we can help you feel a bit better".
You get the picture I hope, even though I am not doing it very well here!
Essentially, anxious kids do best when their feelings are initially validated and normalized, then problem solving together takes place, rather than the parent taking all the "load" or dismissing all the "load" as unimportant or unnecessary.
She gets to make suggestions; you do too; each of you gets to explain why the solution doesn't work.
Eg "how about you dump Malcolm?"
"Well yes, I can see how that would solve the problem from your point of view. But it doesn't work for me. Malcolm is kind, and funny, and I like spending time with him. He is not mean to me, or you, and he is a nice person. So I don't want to dump him. Anyway if I did dump Malcolm you might find you feel the same about my next boyfriend, and I can't just dump every boyfriend forever as I will feel lonely! Let's see if we can keep thinking".