Though retirement can be a fulfilling time in people's lives, it can also be a stressful one.

This especially holds true if you fall victim to the following mistakes, so be sure to avoid them at all costs.

1. Relying too heavily on Social Security

Millions of seniors collect Social Security in retirement, and those monthly payments play a pivotal role in helping beneficiaries keep up with their expenses. But if you're planning to live on Social Security alone once your career comes to a close, you're making a huge mistake.

Contrary to what you may have been led to believe, Social Security isn't designed to replace your former paycheck. If you were an average earner, those benefits will translate into roughly 40% of your previous income. If you were a higher earner, they'll replace an even smaller percentage.

Since most seniors need more like 80% of their former earnings to live comfortably, you'll need to take steps to secure income outside of what you get from Social Security. For the most part, this means funding a retirement plan like an IRA or 401(k) during your working years, but it could also mean planning to work part-time in retirement, renting out your home as a senior, or a host of other possibilities. The key, however, is to recognize that while Social Security will help you pay the bills in retirement, it won't be enough to fund your golden years by itself.

2. Assuming your living costs will drop drastically

Many people assume that once they retire, their living expenses will magically shrink. But chances are, your monthly bills won't change all that much once you're no longer working.

Think about the things you spend money on today, like housing, food, utilities, and clothing. These are all items you'll continue to need when you're older, and whether or not you're working at the time won't really matter. You may even come to find that some of your expenses go up in retirement, like healthcare and leisure.

In fact, the Employee Benefit Research Institute found last year that roughly 46% of households spend more money, not less, during their first two years of retirement, while 33% spend more for their first six years outside the workforce. To avoid financial struggles later in life, map out a retirement budget that accurately reflects the costs you'll face, and make sure the income you anticipate is enough to support it. If not, you might consider postponing retirement until you're in a better place financially.

3. Not taking advantage of catch-up contributions

Many workers fall behind on retirement savings during the earlier stages of their careers, when student loan payments, housing costs, and other expenses eat up most of their income. Thankfully, those who are 50 and older get a prime opportunity to make up for lost years of savings in the form of catch-up contributions.

If you're saving in an IRA and are at least 50 years old, you can currently put in an additional $1,000 each year for an annual total of $6,500 (workers under 50 can contribute just $5,500). If you're saving in a 401(k), you can make a $6,000 catch-up contribution for an annual total of $24,500 (compared with $18,500 for younger workers).

Unfortunately, many folks don't take advantage of catch-up contributions, and as such, wind up falling short by the time their golden years come around. In fact, only 14% of 401(k) participants aged 50 and over made catch-up contributions in 2017, according to data from Vanguard.

If you're behind on savings, it's imperative that you take steps to pad your nest egg, whether it be by cutting expenses to free up cash or taking on a side job and using its proceeds to fund your retirement plan. Otherwise, you may be in for a major disappointment when your golden years arrive and you realize you don't have enough money to do the things you've always dreamed of.

4. Forgetting about taxes

Between your Social Security benefits and your nest egg, you might find yourself on the receiving end of a pretty healthy income stream in retirement, especially if you've saved well. But don't assume all of that money will be yours to keep. Chances are, the IRS will also be entitled to its share, especially if your retirement income is substantial.

There are several ways you might get taxed in retirement. First, unless you have a Roth IRA or 401(k), your nest egg withdrawals will be taxed as ordinary income — meaning your highest possible rate. The same holds true for many types of pensions. Furthermore, if your income exceeds a certain threshold, you could get taxed on up to 85% of your Social Security benefits. Finally, just as interest and investment income are taxable during your working years, so too are they subject to taxes during retirement.

The takeaway? Be sure to factor taxes into the mix when calculating your anticipated retirement income. If you're planning to withdraw $30,000 a year from your 401(k) and you expect your ordinary income tax rate to be 25%, know that you'll end up with only $22,500, and plan your expenses accordingly.

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The more thought you put into retirement planning, the better off you'll be when your golden years finally arrive. Avoid these mistakes, and you'll be setting yourself up for a more financially secure future.