In addition to what others have said about guilt, it's difficult when one's off center, but maybe a few simple things can help.
I like to ask myself this, "How much is any of this going to matter 10,000 years from now?" This world and life, this blip in time we know here, really isn't our home or our life, rather like the birth pangs of the corruptible, we really seeking the incorruptible, in eternity with the Lord, in the Lord's kingdom of heaven: keeping one's focus on the big picture, the prize, our Lord Jesus and eternity, tends to dwarf most everything here. Also, there's a false expectation of happiness in this world that isn't Biblical, that somehow we're to achieve some, as if, "American dream" in the flesh, desiring things that aren't lasting, somehow expecting a sinful world is going to deliver all goodness and light, instead of things which are not even often good, and things not permanent or truly satisfying. False expectations have a lot to do with peoples' unhappiness.
Neither does it ever hurt to venture outside yourself and do something worthwhile, constructive in your own life or, better still, in the lives of others: when you're focused on some worthy pursuit, there's simply not as much time to consider your own problems.
Peace is contentment in the joy of the Lord, thankfulness for all the Lord's tender mercies and salvation, looking towards eternity, looking forward to that truly brilliant life and all the tomorrows in His kingdom, realizing all things in this world and life as we know it pass, both the good and the bad. Counting one's blessings is also appropriate, considering most often that one's fortune is better than many, many others in this world.
Hope this helps you.
2 Corinthians 4:18 While we look not at the things which are seen, but at the things which are not seen: for the things which are seen are temporal; but the things which are not seen are eternal.
2 Timothy 2:10 Therefore I endure all things for the elect's sakes, that they may also obtain the salvation which is in Christ Jesus with eternal glory.
1 Timothy 6:6-8 But godliness with contentment is great gain. For we brought nothing into this world, and it is certain we can carry nothing out. And having food and raiment let us be therewith content.