Retirement is supposed to be an enjoyable period of life, but if you don't plan for it properly, the opposite might end up holding true. Here are four mistakes that could derail your retirement and leave you cash-strapped and miserable at the same time.

1. Relying on Social Security alone

Though Social Security helps millions of seniors stay afloat financially, those benefits aren't designed to sustain retirees by themselves. If you're an average earner, you can generally expect Social Security to replace about 40% of your previous income in retirement. Most seniors, however, need roughly double that amount to live comfortably, which means that if you don't save on your own for your golden years, you're likely to come up short.

A senior man in collared shirt outdoors with serious expression


Remember, though certain expenses, like commuting costs, might go away in retirement, most of your monthly bills will likely stay the same. Some might even go up. Take leisure, for instance. Once you have more free time on your hands, you might start spending more to keep yourself occupied. Similarly, if you're home a lot during the day since you no longer have an office to go to, you might spend more to heat and cool your living space. Be sure to amass some personal savings before kicking off your retirement, because relying on Social Security alone will leave you scrambling to make ends meet.

2. Underestimating your healthcare costs

Many seniors know that healthcare is a major expense they'll need to grapple with in retirement, but a large number fail to realize just how costly medical care can be. Investment giant Fidelity recently shared projections that put the cost of healthcare in retirement at $285,000 for a 65-year-old couple retiring this year.

Of course, your healthcare tab will depend on numerous factors, such as whether you enroll in traditional Medicare versus Medicare Advantage, whether you buy supplemental insurance, and what your actual health looks like. But know this: Fidelity's projections are just one of many estimates out there. HealthView Services, a cost projection software provider, reports that healthcare in retirement will actually cost the average healthy 65-year-old couple today more like $364,000. Therefore, keep tracking the cost of medical care, but just as importantly, ramp up your savings so that you ultimately have the money to pay for it.

3. Forgetting about taxes

Many people assume that seniors mostly get out of paying taxes, but that's far from true. There are a number of ways the IRS might come after your income in retirement. First, if you house your savings in a traditional IRA or 401(k), your withdrawals in retirement will be taxed as ordinary income. The only way to get around this is to save in a Roth IRA or 401(k) instead.

Additionally, your Social Security benefits might get taxed at the federal level if they only make up a portion of your total retirement income. And if you live in a state that taxes Social Security, you'll lose some more money at the state level as well.

Income you collect from a pension is also subject to taxes most of the time (though there are some exceptions to this rule). And if you earn investment income or interest in a nontax-advantaged account (such as a traditional brokerage or savings account), you'll pay taxes on that, too. The point, therefore, is to plan on forking over some money to the IRS year after year. If you anticipate being taxed in retirement, you can budget around it.

4. Not planning for long-term care

Many seniors get injured or experience a decrease in mobility as they age. So, many find that they can no longer manage to live on their own without some sort of help. In fact, an estimated 70% of seniors 65 and older wind up needing long-term care, whether in the form of a home health aide, an assisted living facility, or a nursing home. And if you don't buy insurance to help cover the costs involved, you could be in for quite a financial shock.

The average assisted-living facility in the country costs $48,000 a year, according to Genworth Financial's 2018 Cost of Care Survey. Nursing home care is even more expensive -- $89,297 a year, on average, for a shared room, and $100,375 a year, on average, for a private one.

That's why you really need long-term care insurance going into retirement. The best age to apply is in your mid-to-late 50s, because at that point, you're more likely to snag a discount on your premiums based on your health and age. That said, many seniors apply for policies in their 60s, and if your health is decent, it certainly pays to do so. Otherwise, an extended period of care could effectively wipe out your retirement savings, leaving you broke and vulnerable later in life.

Retirement can be a scary prospect from a financial perspective, but it doesn't have to be. Steer clear of these mistakes, and you'll avoid much of the stress so many of today's seniors face.