Present Bias, Projection Bias
What we want now is often the opposite of what we aspire to in the future. We overestimate how much our future selves will share the same beliefs, values and behaviors as our current selves, causing us to make short-sighted decisions.
Related studies:
Milkman, Rogers & Bazerman (2009). Ice cream soon and vegetables later: A study of online grocery purchases and order lead time.