Can I constantly charge my power bank or does it break it down easily?

It shouldn't but why charge it if you don't NEED to. Sounds like inappropriate usage.

Things To Know About Power Banks:

When you charge a battery, chemical reactions in the cells cause the electrodes to get excited and emit slightly higher radiation in the process of being charged up. The implication is heightened risk of explosion. And you need to avoid that - Can a power bank explode without any triggering factor?
Its normally suggested that the best point to recharge your power bank is between 40% and 60% in order to extend the battery life. This may be true but I don’t think the stress is worth it. This is because…

Most power banks have a life cycle of 500. A life cycle is used up when your power bank is drained from 100% to zero and you recharge again back to 100%. If you only do 50% it doesn't count. So you’ll have to do another 50% to have used up 1 cycle. This is the basis for the suggestion in 2. above.

If you do the maths, it equates to about 1.5 years of optimal performance if you use up your power bank daily. That’s about the same average you get from most phones. After that time you'll need to recharge more frequently.
Nothing lasts forever and I think that 1.5 to 2.5 years is good enough time to enjoy your power bank and charge when you need to, rather than being paranoid about charging it up all the time. And exposing yourself to further risk in the process. There’s more to life than that.
So, sit back, relax and enjoy your power bank and life - safely.