I bought summer clothes and sandals today after I finished my classes. I was running low on summer clothes, so I am happy to have bought them/so I was happy to buy them. But, I paid money, so now I want to earn money!!
From now on, I'm going to do a lot of assignments...
If I made any mistakes, please correct them.
I was happy to buy them: you were happy at the time when you bought the sandals and clothes
I am happy to have bought them: now as you think of your purchase, you feel happy that you have bought them, you do not regret your decision but feel happy about it instead
If you paid an X amount of money, you could say: "I was set back X amount." For instance, if you paid 10,000円 for a dress, you could say: I was set back ¥10,000, and so now I need to work harder to save money.
Using "pay" here in this context is better than using "lose." With "lose" it sounds like you lost money accidentally, like you dropped your wallet in the metro station or someone stole it when you weren't aware, or you lost money in the lottery. But in your case, you made the decision to make the purchase, so you knew when you spent the money on the clothes. :)
Also at the end by "assignments," do you mean school homework or gigs/アルバイト that you want to do to make money? If you want to work to earn money, it's better to use "task" or "gig" or just "work."