How do you know when you're over saving and when to kick back a little and enjoy?

Spend on things you enjoy

Whether $1.5 million is sufficient for retirement at age 40 depends on a few key factors:
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1 Lifestyle and expenses: What is your planned lifestyle and annual living expenses in retirement? Higher expenses will require a larger retirement nest egg. Consider factors like housing, healthcare, travel, hobbies, and other costs.
2 Retirement time horizon: Retiring at 40 means your retirement could last 40-50 years or more. This requires very careful planning and a larger capital base.
3 Investment returns and inflation: Assuming an average annual return of 6-7% and 2-3% inflation, $1.5 million may last 25-30 years in retirement. However, lower returns or higher inflation could deplete the assets much faster. Making use of your CPF funds through investing via the Central Provident Fund Investment Scheme (CPFIS) is something worth looking into
4 Other income sources: Do you have any other retirement income like rental properties? This supplemental income can stretch the $1.5 million further.

When you felt deprived and live on just bread and water on every meal? Once a while treat yourself to something you like, this helps make the saving journey more enjoyable.

Build a separate "free fund", whenever you use money from that to buy something, it's free. Splurge that money on whatever you want.