In a speech on Saturday evening in Mumbai, Dr Singh said that in 2006, 10 percent annual growth looked "eminently achievable" and "the sense of optimism was all pervading".
But he admitted that since then, Indian exports have shrunk and the fiscal deficit has gone up.
"Growth decelerated to 6.5 percent last year and may be only around six percent in the current year," he said. "This has dampened investor sentiment.
"Doubts are being raised in some quarters about the India growth story going astray," he said at a corporate function.
Dr Singh vowed that a raft of reforms announced in September would revive the economy and attract foreign investment, with more policy changes in the pipeline.
"We have dispelled gloom and doom, improved the climate for foreign investment (and) are working hard to restore investor confidence and the growth environment," Dr Singh told business leaders in Mumbai, India's financial capital.
In a strongly-worded speech, the PM said that his government "bit the bullet" when introducing recent reforms, including to the retail sector that will allow global chains such as Walmart and Tesco to open branches for the first time.
The move has attracted fierce opposition, and many Indian states may still act to keep out giant supermarkets to protect small shop owners.
"Some of the steps were considered by many of our critics as politically impossible. We bit the bullet and did what we felt was the right thing to do," Dr Singh said. "Undoubtedly, more needs to be done."
The reforms have already cost the ruling coalition its parliamentary majority with the exit of an allied party that has threatened to bring a no-confidence motion against the government when parliament reopens later this month.
The Congress-led government has suffered a difficult second term in power amid policy paralysis, worsening economic data and corruption allegations, and is looking to revive its fortunes with the next general elections due in 2014.