Friend uses me to offload, how to enforce boundaries

[Sickofinsomia]

I've come to realise that she's always there when she's needed to vent and offload about something, whether that's been her divorce, her mother, her job and so on, it's usually related to men though.
Of course that's what friends are for, we all need a good rant and advice, it's just that she's nowhere to be seen otherwise.
I've known her for a long time, over 25 years, and it's often been the same unfortunately.. the issue is she's currently booked 2 weeks of annual leave as she says, yet is far too busy to even meet for an hour.
I know Christmas and New Year is family-oriented however I am suspicious that she can't find a single slot in the 2 weeks..I haven't seen her in over a year now though so this is nothing new.
The other day she messaged me saying 'Heyy!! How are you?! How's the new car?!'
I could see right through it tbh though decided to give her the benefit of the doubt, however it wasn't long before she started to offload her guilt regarding her current rebound fling. Once she'd done this and I'd reassured her, it was back to shorter messages and being too 'busy'.
I tend to struggle with this in relationships.. need to enforce boundaries in some ways, any advice? Maybe I should seem more disinterested?

Just stop engaging. You’re fully capable of sending shorter noncommittal messages or no messages at all. Do so. You haven’t seen this person in over a year. This is not a friend.

You knew what she was going to do and you let her do it. 25 years. Time for you to be busy!

Yeah you're right I need to, I seem to be that sort of person who likes to help people and I end up offering her constructive replies, but I'm going to make it my resolution to stop! She's part of the same friendship group which makes it a bit awkward, but I guess I don't need to be rude or cold, just more noncommittal as you say

Just be as busy as she is. No need for explanations or boundaries, you are her friend but she is not your friend IYKWIM.

Since this is not a reciprocal friendship or friendship at all, it is enough with blocking her or stoping making yourself available and if she insists just explain why.

Are you getting anything out of the relationship? You say you like helping people. It's okay if you enjoy the 'agony aunt' bit as long as you adjust your expectations that this relationship is going to be more. Invest your actual resources - time, energy and money etc - in relationships and activities that you get benefit from. You have fallen into a groove into this friendship and you can leave it there - you are unlikely to be able to change it's path now and it would doesn't sound worth trying.

Engage for a few minutes then change the subject. Rinse and repeat. Or just take a step back from the friendship altogether. Doesn't sound like you're getting much out of it anyway

I do enjoy helping and I enjoy the conversations but I find myself repeatedly disappointed by her. I will go low contact due to the friendship group but I need to get the strength to either ignore or be non-committal,wish I wasn't such a people pleaser

Agree take a step back. Op I learnt the hard way people suit themselves. Think friendships are 2 way street

Next time she makes contact with you say "Oh that's nice. I'm sorry I can't stay and chat as I'm really busy today. Talk again soon" and then just stop talking or say goodbye or walk away saying bye (depending on whether it is over text or in person or a facetime call).

People will genuinely value you for you and your time if you value your time and value yourself. You are important and not just a sounding board for her to off load to.

I seem to be that sort of person who likes to help people and I end up offering her constructive replies

And therein lies the issue.

If she messages something long-winded about breaking up with some bloke, just reply with a 'never mind, plenty more fish in the sea lol' kind of remark.