Rav Chaim Schmuelvitz suggests that the name Yisrael connotes happiness while Yaakov reflects sadness. He says this is clear at the start of VaYechi: “And it came to pass after these things that someone said to Yoseph: 'Behold, your father is sick.' And he took with him his two sons, Menasheh and Ephraim. And someone informed Yaakov, and said: 'Behold, your son Yosef is coming to you.' And Yisrael strengthened himself, and sat upon the bed.” (There’s an interesting literary parallelism – first someone tells Yosef something about his father, then his father is told something about him. Wonder what the message is in that little piece of Torah poetry.)
First sad, then reinvigorated; Yaakov then Yisrael. This reminds me of an interview I saw in which Robert Klein was asked if he sees himself or someone else when he watches videos of his old performances. He said that he actually sees different people, not him as he knows himself now. We are all (hopefully) different people during different ages, different moods, different times.
Yaakov was sometimes so sad that he was a different person. And yet he was always forefather and role model. Different states of mind are opportuniteies, not excuses. This is a difficult truth, true nonetheless. Wherever we're at and whoever we are at any moment we are expected to be our best, to do to our best to be close to G-d.
May we be so blessed.