Friend wants to meet up but I’m an after thought

There's not really enough information here to give a full response but from what you've said I think you should give your friend a chance.
She is coming to your area and made the effort to make plans with you, not as an afterthought, just separate from other friends.
If you've lost previous friendships to MH issues, I would try and keep the people who stuck with you in the tough times. Even if they weren't there constantly, this person is there now and reaching out.

Sometimes, it's not even about something upu have said/not said but more about the 'energy'.

People want to spend time with people who make them feel good. Good about themselves, good about life, just energised and revitalised. How we all want to feel after seeing friends and why we look forward to seeing them.

OP, if one person's energy is in conflict with then others, then no one gets out of the friendship/meetings what they want/need.

This is important. Given many of your other friends don't have anything to do with you now I'm wondering what communications you had. Not saying you must have said something that made them cut contact but I'm wondering if you did. In which case it's admirable this friend is making contact

Why can't she see both set of friends? You're not a afterthought she's literally made plans with you?

I wouldn’t go and I’d let it fizzle out. You may find she cancels after her night out.

Personally, I think I would go. But I'd also try and find childcare so that your catch up is just the two of you. Two old friends, two women, two adults without a toddler in tow.

since I have had my 3 year old daughter this friend has seen me once and made no effort to communicate ie avoided calls, texts etc and only met my daughter when she was 2 last Xmas.

On the surface, thissounds tough. I'd take some time.to reflect on the nature of your communication during this time.

Were they light hearted catch ups with a friend or were you seeking MH support?

Not expecting you to answer but just reflect.

Like I said, I've been the person on both sides of this dynamic.

When I was 'bad' I had no idea that I was asking things of others that they were unable.to give. I thought I was just getting in touch with a friend but, when I reflected afterwards, I really wasn't. My normal, my life and my day to day was actually very far from other people's and I was actually asking a lot of them mentally (not physically or practically) that they just couldn't give mainly - even just listening - because I just felt very negative and was dealing with a load of shit.

Take a step back. If you don't want to meet with her, don't. But, if your MH is in a much better place there days, maybe it's a opportunity to draw a line and find a new friendship with her.

So, friend cannot plan to see more than one group of friends on the trip? Only a journey planned to see you specifically and not other people is acceptable? YABU

If this friendship isn't bringing anything to your life then I point investing on it.

It sounds like she wants to see you and has combined two things into the weekend.

There's clearly a lot going on for you. MH issues that are significant enough to cause all your other friends to withdraw from the friendship must have been pretty significant.

Previously, she might have had reason to believe it was too much pressure for you or, in the kindest way, just a lot of effort for a weekend that risked not being an enjoyable weekend for her. Even close friends have their own time, life and MH to consider.

When you have significant MH needs, you're often unaware of how this impacts on others around you (speaking from experience from both sides here).

She clearly wants to see you to suggest it but, as the pp said, it's up to you whether you want to meet her or not.

I'd say you'd probably be burning a bridge with that particular friend if you don't though.

From the information you have given there is nothing to say you are an afterthought. She is travelling a long distance and had arranged to meet you. It makes perfect sense to combine seeing two friends when making a long journey.
She clearly does want to see you as has reached out to you. Completely up to you if you don’t feel the friendship has been there over the past few years.
I wonder if this is your mental health talking ?