Abundant means existing or occurring in large quantities; plentiful.
Fulsome means flattering to an excessive degree; can also mean abundant.
These two words are adjectives, often considered synonyms. Both mean something similar to “having a lot of something”. “A lot” in this case meaning “more than enough.” Some people may use them interchangeably, but there are some differences.
Abundant is more likely to be used to describe the actual stuff that you have a lot of:
Our dinners came with abundant bread on the side.
The library contained abundant books.
Fulsome ,on the other hand, is more likely to describe the thing that is full of abundant stuff. In other words, its meaning is very close to “full”:
The dinner was fulsome with bread and laughter.
She had a fulsome smile.
Like “full,” fulsome can sometimes mean “over-full” or as we say “full of it,” in a bad way:
He’s a fulsome flatterer; he overdoes it
To summarize, abundant and fulsome can be used interchangeably, although abundant usually refers to the stuff that there is a lot of, while fulsome means “full of abundant stuff” and can sometimes mean “too much.”