Why the sudden change of heart (Muslim Brothers and Sisters)

ٱلسَّلَامُ عَلَيْكُمْ
I hope you are doing well my brothers and sisters. Sending my Duas to the Muslim Ummah, Hoping everyone in the strongest of Iman and Health.
I met this girl, 3 years ago back and everything clicked between us, everything was relatable and understandable and my intentions were pure and well suited around her needs I only told my mum of my intentions of marrying her and wanting to do my nikkah at the time she told me she was not ready to get married until her education at university was complete coincidentally it’s coming to an end this coming April and up till now I have waited for her patiently, although she tells me now after that long of waiting for her financially or mentally she is not ready for marriage, I don’t understand it’s not as though she will be paying the mahr (dowry) or providing towards a wedding it’ll be me and myself and whilst I respect her mental wellbeing comes first I totally understand and closed the door on the subject although I’d like to know do you think there’s someone else lined up for her especially through arranged marriage? She has spoken one or two times bringing it up about marriage to her dad and her dad has said to get married I would need to have a stable job I acknowledge in this day and age the job market is challenging as it is and I have so far struggled to find my degree job although I have shown and set examples of providing for my family single handedly and with a good source of income. What do you reckon Brothers and Sisters?

Walaykum Assalam, maybe speak to her dad directly so you have a more clear understanding on what her family is thinking.

I reckon you haven't done enough to convince her and her parents that you are the right man for her to marry.
Either because you actually aren't the right man for her to marry.
Or because of a failing of "marketing" on your part where you haven't "marketed" yourself well enough to them.
Or maybe a combination of the above 2.
Making the breakthrough to marriage will involve fixing the above. Which to some extent will involve you cleaning the "poisoned well" and therefore be more challenging than if you were starting with a clean slate with someone.
Getting a job where you're looking after horses (metaphorically or literally) will be doable if you're the sort of calibre of man that's right for her to marry. Get good at shovelling ****...

Walaykum Assalam, maybe speak to her dad directly so you have a more clear understanding on what her family is thinking.

Having such a conversation is too likely to cement in the father's mind that the original poster isn't the right man to marry his daughter.
Better to clean out the poisoned well without asking the well owner how to clean it.

Why am I not though? How have I come across bad for his daughter? I have been there supporting her and helping her when she needed me most…. I waited patiently and respectfully as she wanted… and yet 3 and a half years down the line she tells me? Whilst I respect it I still want to know my position and where I stand because I love her for her and the sake of Allah and I want to marry her.

From the opening post, you told us that you've not been able to get a stable job.
There's something the stable owners are seeing in you that has resulted in them offering the job to other candidates.
There will be something about you or your marketing of yourself that's causing this.
There's a reasonable chance that what this is that's holding you back in these job applications is also a large piece in the jig-saw as to why your fiancee and her parents haven't been begging you to marry her.
It's impossible for me to say for sure what you should focus on first because I don't know you well enough.
I can speculate as to the most likely things you should focus on.
Your social skills.
Your self auditing skills
You skills in fixing whatever's thrown up by your self audits.
Your shovelling **** skills and ability.
Being a white knight isn't good enough for you to marry her. You yourself have to be a desirable man and you have to demonstrate to them that you're a desirable man.
Their desire when it comes to you marrying her, right now, may well be down to them having the right feelings and emotions towards you. White knighting isn't the best way to get them into the right emotional state about you. You being something that they want and desire is. And by that I mean you pushing the right emotional buttons.
Buyer-seller dynamic.
Ideally, they'd be in a position where they feel that they have to win you over in order to get you to marry her.
Right now, it seems that they don't have that feeling. They feel that it's you that's trying to win her and her parents over.

I'm speculating here. It may not be what you've tried. It's how you've gone about trying it that may be the issue.
How you come across at interviews.
This is one of those things where if I were to sit in on the interview sessions that have included you as a candidate, it would probably be blindingly obvious why other people were offered the job and you weren't.
And if I were an observer at the times you've interacted with your fiancee and her parents, it would probably be obvious why they're stone-walling on the marriage.
You may well be close to being the finished article. And it may be a case of a few tweaks here and there to get you over the line.

I wonder what it is I must do then?

From the opening post, you told us that you've not been able to get a stable job.
There's something the stable owners are seeing in you that has resulted in them offering the job to other candidates.
There will be something about you or your marketing of yourself that's causing this.
There's a reasonable chance that what this is that's holding you back in these job applications is also a large piece in the jig-saw as to why your fiancee and her parents haven't been begging you to marry her.
It's impossible for me to say for sure what you should focus on first because I don't know you well enough.
I can speculate as to the most likely things you should focus on.
Your social skills.
Your self auditing skills
You skills in fixing whatever's thrown up by your self audits.
Your shovelling **** skills and ability.
Being a white knight isn't good enough for you to marry her. You yourself have to be a desirable man and you have to demonstrate to them that you're a desirable man.
Their desire when it comes to you marrying her, right now, may well be down to them having the right feelings and emotions towards you. White knighting isn't the best way to get them into the right emotional state about you. You being something that they want and desire is. And by that I mean you pushing the right emotional buttons.
Buyer-seller dynamic.
Ideally, they'd be in a position where they feel that they have to win you over in order to get you to marry her.
Right now, it seems that they don't have that feeling. They feel that it's you that's trying to win her and her parents over.