I suspect that there's an issue here with your self esteem that's making you put her before your own needs. Women and girls are 'trained' from very young g to,be compliant and to people please and it's a hard habit to break. That isn't simple to fix and won't happen overnight, but you can make a start.
Firstly by listening to all the previous posters who have validated that it's OK for you to feel used and frustrated, and that it's fine (and normal) to put yourself and your needs first- ahead of what your friend is asking. You need to start really believing this.
Secondly, start to schedule in, and (re) discover things, that make you happy. This will reinforce the feeling that your life has purpose and that you can be happy. And that your time is valuable -to you.
Thirdly, practice and read about assertiveness, it really is life changing. If you can, role play a situation that you currently find difficult, like a request from your friend, or someone else. The rule is this (and it's so hard at first!) the ONLY word you are allowed to say in your initial response is 'NO'. Nothing else, no justification, no explanation. (it's cringingly difficult to do,at first!). For example
Can you feed my cat whilst I'm on holiday?'
NO
Oh, but I need you to, you're the only one I trust
NO, I still can't do it
But who else can I ask? You always do it
Well, you'll have to find someone else as I can't do it.
But why can't you?
Dear friend , I know you are used to me saying yes to all the many favours you ask of,me, but this time its a firm no.
But why?
I really don't need to go into all the ins and outs, please find someone else to,ask as I won't be doing it.
And so on. Never explain. Never justify stay calm, breathe through it . Don't offer solutions to her predicament either as she will reject them and out you back in the hot seat.
If she carries on pestering, change the subject 'so how's work going at the moment?' Or something innocuous. And focus on that new line of conversation.
If she still keeps on, end the conversation completely. 'Well, is that the time? I really must be off, nice chatting to you, cheerio' and leave or hang up.
If you really want to you could add on.
'Dear friend, I need to be straight with you. I've been feeling totally overwhelmed recently, and you've been asking a lot of me on top of what I have in my own plate. I just can't keep saying yes to you. It's killing me. I'm going to start saying no to many of the things you ask of me. I don't think any less of you. I need to put my needs first. I know that you'll understand and will stop asking me so often; even the little things add up and I can't agree to do them. I really appreciate your support.
Dot t say: maybe, or I'll think about it, or I'll check my diary, or sorry, or maybe next time.
But as I suggested at the start, working on your self esteem will really help,in this and many other areas of your life. You are worth it, you really are. You deserve a life of happiness and doing things that bring you joy and satisfaction. You weren't put on this planet as a servant of others. Your time is precious, spend it wisely.
And the really magical thing about boosting your self esteem is that the more you like and love yourself, the more,other people around you do too. It's hard to,get there and takes work to maintain, but it's the single thing you can do for,yourself,that's truly life changing. Go for it!