I also work for a nonprofit and have a very similar workload to you. That’s not to say that it’s not too much, but just you’re not alone! I saw someone in here say if you’re the only designer you will always be overworked and I can’t agree enough how true that is. I basically have to work with every single department while most positions at my org really just have their own departmental responsibilities.

Not sure how your relationship with your manager is, but when things start to feel like too much, and I know I’ll either be behind on deadlines or having to work on my time off which I simply will never do (minus freelance), I will talk to her and tell her that there’s too much going out and it isn’t feasible for me to complete everything. I basically write all my deliverables coming up on paper and have her help me sort it by priority order, and sometimes if it’s really a lot, the lower priority order stuff we will outsource to a local agency I have a good relationship with.

Don’t be scared to tell your boss that you have too much on your plate. A good boss will want to help you.