Does anyone have any advice on how to not stay discouraged? I really want to go to graduate school, but everything is so up in the air currently I just get stressed about the idea of graduating and not being able to find anything in my field.

One of the most important skills to build for grad school and beyond is not getting discouraged in the face of a lot of discouraging stuff. At every step of the way you will experience manuscript rejections, failed experiments, unsupportive advisors, writers block, burnout, etc. and although it is frustrating and crummy, having a thick skin and moving past it is really important to your success. So, if you really want to go to grad school, don't dwell on your advisors criticisms too much. Also, pay attention to the criticism and see if there are tangible things you can improve on. Sometimes it is really tough to pull the constructive criticism out of what feels like a personal attack, but remember advisors are not perfect and likely don't realize how their criticism is coming across.

I just feel like her criticisms of me and my work has made me feel like it'll only get harder if I try to do a thesis based grad school program.

Grad school almost certainly will be harder than your work in undergrad, but it is a completely different environment. I know many people that struggled in undergrad and end up thriving in the grad school environment (and vice versa). Seriously evaluate why you want to go to grad school, and decide if the commitment, challenge, etc. is worth what you would like to get out of it.

But would all grad schools be like this?

It heavily depends on who you end up working with and what the lab environment is like. There is no shortage of advisor horror stories out there, and there are truly some toxic advisors. On the other hand there are many extraordinary advisors as well. It is really important to try and gauge this during interviews and lab visits.

I've just felt very discouraged about finding funding for environmental science (I'm from the US btw) because of lack of interest in my field from the public

Yeah there is no way to sugar coat this, but ecotox funding prospects in the US are pretty bad at the moment. The EPA did open up Star Grant funding again though, so I'm cautiously optimistic that the situation will improve. Finding opportunities might be challenging, but don't let it discourage you from applying. If you know the type of project you would like to do, it is a good idea to start searching early. Check job boards, and pester your advisor to put in a good word for you in her network.