How to Fight Imposter Syndrome as a student by myself?
Even with proper psychological treatment at your university, imposter syndrome (IS) is tough to deal with. I agree with @erwan: recognizing you experience IS is the first step.

I suffer from IS, and can only make recommendations based on my experience and what has worked for myself and those close to me. I highly recommend joining or fostering a community of those who experience IS (this can be online or in person, and can comprise students, faculty and staff!). Organizing groups to talk about IS, or even chatting with a fellow grad student over coffee about IS will likely yield revelations. There are probably some students in your department, or lab, who think that you really have your shit together, when you think you don't! 

If you're on Twitter, I have found it a great place for IS. Following people (post docs and faculty) who publicly post about how they are handling their IS (and mental health issues) has been great for me--when I see someone who I consider successful posting about how they don't feel they are good enough for X, or how they don't think they can complete Y makes me think, 'if they are suffering from IS then surely I can excuse myself!'.

Is there a cure for IS--imo, no. What would you have to do, or what goal would you have to achieve to consider yourself successful? Is this an attainable goal (by yourself or others)? When you think about a peer or mentor who you consider successful, how do you measure that and why do you use those measures? Is it fair to compare yourself to others? 

Briefly,


Join or create a local community to discuss issues with IS in academia (this group could meet regularly, or not!).
Remember that you're not alone in dealing with IS -- follow people who talk publicly about IS/issues in academia on social media.
Don't define yourself by only your academic achievements--you're more than that.
IS doesn't have a cure, rather, you'll have to employ coping mechanisms to ameliorate or prevent the symptoms.
Find a way to measure and celebrate your successes.