I want to say, "I was overwhelmed by the workload." I see the word überfordern but I'm not sure if that's correct. Can I say, "Ich war von der Arbeitsbelastung überfordert."?
The verb "to Overwhelm" is "überwältigen" or "übermannen" in German, but in some situations it also can be "überschütten" or "überhäufen":
Lisa was overwhelmed with emotions when she saw her mother again after 30 years.
Lisa wurde von Gefühlen überwältigt/übermannt, als sie nach 30 Jahren ihre Mutter wieder sah.
The clerks were overwhelmed with inquiries after the management published the new policy.
Die Angestellten wurden mit Anfragen überschüttet/überhäuft nachdem die Geschäftsführung die neue Richtlinie herausgegeben hatte.
The German verb "überfordern" is "to overcharge", "to overburden", "to swamp" or "to overstrain"
Ich war von der Arbeitsbelastung überfordert.
I was overburdened by the workload.
"Überwältigt sein" means that something causes an surprising high amount of emotions, but that doesn't mean that you can't deal with it, and even more important: It has no negative connotation. In my first example Lisa was extremely happy wo see her mother again. This is a very positive emotion, and Lisa probably was able to deal with this emotions, but still to see her mother again was a much stronger sensation than she would have expected.
"Überfordert sein" means that something did rise over the limit that you can deal with. It is too much, you can't handle it any longer. And it has a strong negative connotation. It can mean, that someone is too week, or that the load was too high:
Der Neue ist ja schon mit den einfachsten Aufgaben überfordert.
The newbie is overburdened even by the simplest tasks.
Das ungeheure Ausmaß der Katastrophe überforderte sogar die bestens vorbereiteten Einsatzkräfte.
The enormous extent of the catastrophe even overburdened the well-prepared emergency services.
Überfordern is somewhat dangerous to use, since it may suggest, that one were not sufficiently qualified for the job.
The most neutral term is überlastet, which simply means übermäßige Belastung and which neatly scales down to ausgelastet (enough work assigned) or nicht ausgelastet (could manage additional tasks). Since überlastet does not combine well with Arbeitsbelastung due to the same stem, I propose:
Ich war durch das Arbeitspensum überlastet.