Probationary employees in Ontario must be given reasonable opportunity to demonstrate their suitability for a job and must be treated fairly. This means they can't bench you for 2 months and then give you the boot--they need to give you a reasonable amount of work, which forms the basis on which you are judged as a probationary employee.
Also, afaik employees are generally not entitled to any notice of termination or pay in lieu of notice unless their contract states otherwise. Does your contract include such an entitlement?
Unsuitability is kind of a wishy washy term that can encompass any number of things, but you not being a good "fit" is reasonable grounds for termination. Your employer must be reasonable and fair in its dealings with you, but the whole point of probationary periods is that they give you and the employer a set period in which to figure out whether or not yall actually get on and like each other. If they decide it's not a fit, and they reach that conclusion fairly, then it is what it is.
You got almost the entire 90 days to prove yourself, during which you received an appropriate amount of work (if I'm understanding the situation correctly) and were otherwise treated fairly. As such, we can't say they made the decision prematurely.
Your illness isn't grounds for termination, but how you handle it is... a bit more of a grey area. Did you contact the right people to discuss leaving early/taking a sick day? Did you follow company procedure? Did you tell them what the medication was? (If you did, don't ever do that again--your employer is not your friend.)
And you also really don't know just how you were received by management. I've met some amazing law students who were brilliant during their articles but just didn't fit in their firms. They were not re-hired as a result. They were responsible for some pretty great work products, sure, but a big part of any job is how well you fit in the environment and what the people around you think. They don't hire people on the basis of work product alone. You spend most of your weekdays at work with the same people--you can't just look at work product and ignore the rest. Unfortunately for you, it just didn't work out.
As for coaching--what were you promised in your contract? What is standard at the office? You can't just walk in and demand mentorship from whomever. If someone says they're busy, then they're busy. You asking for feedback repeatedly from the same busy person may well have contributed to the fit issue.
Your employer has obligations to you, yes, but there are many ways of meeting those obligations that do not involve direct mentorship. Unfortunately, those obligations do not require that they hold your hand. Are there company policies about performance reviews prior to termination? About warnings? About performance reviews more generally? If they acted in good faith and followed their own policies, then there's not much you can do. They aren't required to expend a huge of time and effort in you, especially if you're a probationary worker. They just need to treat you fairly (the same as anyone else, unless specific accommodations have been discussed) and give you reasonable opportunity to show your stuff. It sounds like they did both.
And yeah, it sucks. Capitalism generally sucks, especially when you have mental health issues to wrestle with alongside the demands of a job. But it doesn't sound like they did anything untoward unless you have documentation showing what they should have done otherwise.
Take the weekend off. Eat some turkey. Relax if you can. Then start up the job hunt again.