Ramban asks this question (thanks @Chatzkel) and explains that Rashi has made a mistake, and the real interpretation of this verse is as follows:
והחסדים הם הטובות שעשה עמו בלא נדר והאמת הטובה אשר הבטיחו ואימת לו
הבטחתו יאמר שאיננו ראוי שיבטיחנו ויעשה לו אותן טובות שהבטיחו בהן ולא
לטובות אחרות רבות שעשה עמו
Now hachasadim (the mercies) are the kindnesses which G-d did for
him without having vowed to do them, and ha'emet (the truth) is the
kindness which He promised him and fulfilled. Jacob thus said that he
was unworthy of G-d’s promising him and performing those kindnesses
which He promised him, nor was he worthy of those other many
kindnesses which He did for him without having promised to do them.
Basically, when Yaacov says he was unworthy of the "truths", he was saying that he was unworthy of the promises in the first place.
He then adds that Emet is referring to long-lasting kindnesses that are well established, such as children.
My own thought: One might thus say in defence of Rashi that Rashi could be saying that Yaacov was humbled by the way Hashem kept His promises. As יהושע ק@ pointed out, Yaacov was Hashem's Chariot because of his having taken truth in purity (see Shaarei Orah Hakdama 6), so therefore he was an expert in "truth". His praise that Hashem's truth is humbling could mean that the way Hashem keeps His promises makes him feel small i.e. 'I thought I was good at keeping promises but Your promise keeping is truly Divine; You promised me children, but I didn't expect to be given such wonderful children, so many children, and a family line that will never persih' etc.