Answer by taking a quote from the following article:

Philip Alfred Mickelson (born June 16, 1970), nicknamed Lefty, is an American professional golfer. He has won 43 events on the PGA Tour, including five major championships: three Masters titles (2004, 2006, 2010), a PGA Championship (2005), and an Open Championship (2013). Mickelson is one of 16 players in the history of golf to win at least three of the four majors. He has won every major except the U.S. Open, where he has finished runner-up a record six times.

Mickelson attended Arizona State University in Tempe on a golf scholarship and became the face of amateur golf in the United States, capturing three NCAA individual championships and three Haskins Awards (1990, 1991, 1992) as the outstanding collegiate golfer. With three individual NCAA championships, he shares the record for most individual NCAA championships alongside Ben Crenshaw. Mickelson also led the Sun Devils to the NCAA team title in 1990. Over the course of his collegiate career, he won 16 tournaments.  Mickelson was the second collegiate golfer to earn first-team All-American honors all four years. In 1990, he also became the first with a left-handed swing to win the U.S. Amateur title. Mickelson secured perhaps his greatest achievement as an amateur in January 1991, winning his first PGA Tour event, the Northern Telecom Open, in Tucson. At age 20, he was only the sixth amateur to win a tour event and the first in over five years after Scott Verplank at the Western Open in August 1985. Other players to accomplish this feat include Doug Sanders (1956 Canadian Open) and Gene Littler (1954 San Diego Open). With five holes remaining, Mickelson led by a stroke, but made a triple-bogey and was then three behind. The leaders ahead of him then stumbled, and he birdied 16 and 18 to win by a stroke. To date, it is the most recent win by an amateur at a PGA Tour event.  That April, Mickelson was the low amateur at the Masters Tournament in Augusta, Georgia. With his two-year PGA Tour exemption from the Tucson win, he played in several tour events in 1992 while an amateur but failed to make a cut.  Mickelson's first major championship win came in his thirteenth year on the PGA Tour in 2004, when he secured victory in the Masters with an 18-foot (5.5 m) birdie putt on the final hole. Ernie Els was the runner-up at a stroke back; the two played in different pairs in the final round and had traded birdies and eagles on the back nine. In addition to getting the "majors monkey" off his back, Mickelson was now only the third golfer with a left-handed swing to win a major, the others being New Zealander Sir Bob Charles, who won The Open Championship in 1963, and Canadian Mike Weir, who won The Masters in 2003. (Like Mickelson, Weir is a right-hander who plays left-handed.) A fourth left-handed winner is natural southpaw Bubba Watson, the Masters champion in 2012 and 2014.  Just prior to the Ryder Cup in 2004, Mickelson was dropped from his long-standing contract with Titleist/Acushnet Golf, when he took heat for a voicemail message he left for a Callaway Golf executive. In it he praised their driver and golf ball, and thanked them for their help in getting some equipment for his brother. This memo was played to all of their salesmen, and eventually found its way back to Titleist. He was then let out of his multi-year deal with Titleist 16 months early, and signed on with Callaway Golf, his current equipment sponsor. He endured a great deal of ridicule and scrutiny from the press and fellow Ryder Cup members for his equipment change so close to the Ryder Cup matches. He faltered at the 2004 Ryder Cup with a 1-3-0 record, but refused to blame the sudden change in equipment or his practice methods for his performance.  In November 2004, Mickelson tallied his career-low for an 18-hole round: a 59 at the PGA Grand Slam of Golf at Poipu Bay Golf Course in Hawaii.  The following year, Mickelson captured his second major at the PGA Championship at Baltusrol, in a Monday final-round conclusion that had been forced by inclement weather the previous day. On the 18th hole, Mickelson hit one of his trademark soft pitches from deep greenside rough to within a 18 inches (460 mm) of the cup, and made his birdie to finish at a 4-under-par total of 276, one shot ahead of Steve Elkington and Thomas Bjorn.  Mickelson captured his third major title the following spring at the Masters. He won his second green jacket after shooting a 3-under-par final round, winning by two strokes over runner-up Tim Clark. This win propelled him to 2nd place in the Official World Golf Ranking (his career best), behind Woods, and ahead of Vijay Singh and Retief Goosen.  After winning two majors in a row heading into the U.S. Open at Winged Foot, Mickelson was bidding to join Ben Hogan and Tiger Woods as the only players to win three consecutive majors (not necessarily in the same calendar year). Mickelson was the joint-leader going into the final round, but he was part of a wild finish to the tournament, in which he made major mistakes on the final hole and ended up in a tie for second place at +6 (286), one shot behind Geoff Ogilvy.  Mickelson bogeyed the 16th hole of the final round. On the 17th hole, with the lead at +4, he missed the fairway to the left, and his drive finished inside a garbage can, from which he was granted a free drop; he parred the hole. He had a one-shot lead and was in the last group going into the final hole.  Needing a par on the 18th hole for a one-shot victory, Mickelson continued with his aggressive style of play and chose to hit a driver off the tee; he hit his shot well left of the fairway (he had only hit two of thirteen fairways previously in the round). The ball bounced off a corporate hospitality tent and settled in an area of trampled-down grass that was enclosed with trees. He decided to go for the green with his second shot, rather than play it safe and pitch out into the fairway. His ball then hit a tree, and did not advance more than 50 yards (46 m). His next shot plugged into the left greenside bunker. He was unable to get up and down from there, resulting in a double bogey, and costing him a chance of winning the championship outright or getting into an 18-hole playoff with Ogilvy.  After his disappointing finish, Mickelson said: "I'm still in shock. I still can't believe I did that. This one hurts more than any tournament because I had it won. Congratulations to Geoff Ogilvy on some great play. I want to thank all the people that supported me. The only thing I can say is I'm sorry." He was even more candid when he said: "I just can't believe I did that. I'm such an idiot."

How did he feel about his loss?
Mickelson said: "I'm still in shock. I still can't believe I did that. This one hurts more than any tournament because I had it won.