antonym for "look forward to"

'Apprehensive'1 or 'Apprehend'2 may be the word you're looking for.

E.g. She was apprehensive about the next two weeks of the tournament.

Or "She became/came to be apprehensive about the coming weeks of the tournament"

Alternatively, "She came to apprehend the two weeks of a major tournament"

Though I tend to generally avoid the use of the word in this form as it can easily be misunderstood as the verb of a different definition

Adjective meaning to feel uneasy, anxious or fearful about something.

I would say that the antonym of look forward to would be have reservations about:

She came to have reservations about the two weeks of a major tournament.

From Merriam-Webster's definition of reservation:

2 b : DOUBT, MISGIVING
// had serious reservations about marriage

Of course, have misgivings about could also be used.

If you need a single word, then:

She came to question the two weeks of a major tournament. OR
She came to doubt the two weeks of a major tournament.

Consider anxious:

Feeling or showing worry, nervousness, or unease about something with an uncertain outcome.
‘she was extremely anxious about her exams’
‘an anxious look’
_{Oxford Living Dictionaries}

For your usage:

She was anxious during the two weeks before a major tournament.

Regret[ ri-gret ]

verb (used with object), re·gret·ted, re·gret·ting.

1) to feel sorrow or remorse for (an act, fault, disappointment, etc.): He no sooner spoke than he regretted it.

Source: Dictionary.com

"His sister, not yet a tennis player, would come to regret the two weeks of a major tournament."