The British number one, who also lost in the first round in Indian Wells last week, went down 1-6 7-5 6-4 to the world number 50.
Watson has not won a match since reaching a career-best ranking of 39 in the world.
"This year I've been going a bit too long so I'm a bit burnt out," she said after the match.
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"I've been thinking about it for a while now and I'm definitely going to take a break from tennis."
However Watson has now changed her tune, telling the BBC that she would merely return to her home in Guernsey for a few days.
"I'm going to carry on with my schedule," she said. "I think after my match I just wasn't thinking straight and said things that could be interpreted in the wrong way.
"When I said I was burnt out and wanted to take a break, I meant I wanted to go home to Guernsey for a few days.
"I haven't been home to Guernsey since I left in December. I basically just want to put my racquet down for three or four days, have a rest and see my friends and family, as I haven't done that this year yet.
"I'll still play my normal schedule - Charleston in a week and a half, and the Fed Cup.
"This was my second year with the team getting to the play-offs and I felt so confident in our team of getting there. I'm looking forward to playing against Argentina, in Argentina."
The WTA event in Charleston, South Carolina, starts on April 1 while Great Britain's Fed Cup World Group II play-off against Argentina will take place in Buenos Aires on April 20-21.
Watson, 20, had no problems in the first set against an opponent ranked 11 places below her, breaking her twice from seven opportunities.
Her service let her down in the second set, with Morita breaking her three times, the final time at 5-4 up, with Watson also breaking twice.
The decider was a close affair, with Morita converting her third match point to seal a match against 31st seed Yanina Wickmayer in the second round.
"I was looking at the stats and I actually won more points, but that can happen in tennis," Watson added.
"It really hurts, I think I just need to find myself, find my game again.
"I was playing really well, probably the best I've played this year, and I think I lost my concentration a bit, but that was enough for her to gain some confidence and step forward, and it kind of pushed me back and I started to play a lot worse.
"I still tried to fight but it just wasn't good enough, I wasn't aggressive enough."
Despite her rise up the rankings, Watson is yet to beat a player ranked higher than her this season.
WTA Miami results (Tuesday)
Round 1
Andrea Petkovic (Germany) beat Bojana Jovanovski (Serbia) 6-3 6-1
Kimiko Date-Krumm (Japan) beat Chanelle Scheepers (South Africa) 6-2 6-0
Peng Shuai (China) beat Sofia Arvidsson (Sweden) 6-3 6-2
Kiki Bertens (Netherlands) beat Anna Tatishvili (Georgia) 6-0 1-0 RET
Flavia Pennetta (Italy) beat Johanna Larsson (Sweden) 6-4 6-1
Irina Begu (Romania) beat Marina Erakovic (New Zealand) 6-1 4-6 6-4
Kristina Mladenovic (France) beat Aleksandra Wozniak (Canada) 7-5 6-2
Ayumi Morita (Japan) beat Heather Watson (Britain) 1-6 7-5 6-4
Hsieh Su-Wei (Taiwan) beat Lesia Tsurenko (Ukraine) 6-4 7-6(15)
Ajla Tomljanovic (Croatia) beat Ksenia Pervak (Kazakhstan) 6-2 6-2
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