Or open a Roth IRA account at a discount brokerage (Fidelity, Schwab, Td Ameritrade, Vanguard are the big guns). Then link a checking account to it. Set up free automated deposits; you can tell the brokerage how often to ping your checking account and pull money into your Roth IRA. Click here for more about young people and the virtues of a Roth IRA: https://www.rate.com/research/news/youth-compounding-roth-IRA
Automation is the key. Leave it to your best intentions, and you will spend the $100. That's just you being human.
If it's honestly too much cash to put away, you can always turn off the automated savings (or downshift to $50 a week, or whatever's doable). Just give it at least three months to see if it becomes easier/second nature.
Before you say no, consider whether lifestyle creep is the problem. The average monthly payment for a new car loan is more than $530. Need a car? Understood. You can read more on the benefits of choosing a reliable used car. That might save you $200 a month, leaving you with only another $7 or so a day to reach $400 a month.
Is there a side gig that would help you start saving now rather than later?
And yes, you may need to rethink where you live. It's understandable to want to have a nice place, more amenities, fewer roommates. Are there palatable trade-offs you can live with? A slightly longer commute to save on rent?
You've got more than a million reasons to consider ways to get compounding working for you ASAP.
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