A lot of depression often arises from the never ending stream of tradeoff decisions that comes with life. Some people are depressed by the fact that you cannot have everything you want, they are depressed by their limitations. Others accept the fact that they have limitations but are depressed at the option they are giving up when they make a decision to spend their limited time on one activity (like a movie) and miss out on another (like bowling).
Unfortunately, there exist contradictions within reality which you cannot overcome, and all you can do is come to terms with them. To come to terms with them:
Overcome Perfectionism to let go the Denial of reality (let go the denial of not being to have all you want) according to the Life Education Curriculum. Then, now that you accept an imperfect reality, which is the only true reality we share, use the lessons in Self Improvement Part 2: Planning to maximize what you want given your constraints. As for overcoming the feelings of regret, refocus your attention on the present reality that you are living rather than the imaginary world you are deluding yourself with and appreciate the reality that you have and what you have in that reality.