I feel the same way a lot, but I realize that the negative feelings are what I associate with my life, not a true reflection of how my life has been.
Maybe I haven’t done all the things I’ve dreamed or used my days effectively; why does that matter in the end? I’ve been lucky enough to afford a home, food, a loving pet, create friendships, experience love.
Also I tend to underestimate the accomplishments I have done. Having your eyes set on the horizon can cause you to miss the shells in the sand.