For reference, here's what you did write:
Attendi is either the first person perfectum I have attended or a second person present or imperative, you attended or do attend!. Sum is just 'I am'. Since you can't have two finite verbs in a sentence, I would think there's some ellipsis involved like "I have attended [for] I am".
Latin verbs don't need an explicit subject: veni means I came, not just came. In case "ego" is included as a subject (and it is, because ego is the first person nominative), it means to convey emphasis. However, abierunt means "They have left', so the subject and verb don't match.
All in all, it sounds like a small child who is upset that everybody left when they came, and is having trouble talking while crying at such indignity:
I came, I have attended... I am... (sniffle) They left I!"