We all have our fair share of healthy and unhealthy relationships, namely with friends, family, or significant others. It's quite possible that by nature, we're just very nice people (or try to be nice) so that we can please everyone and improve our relationships with people we care about. It's also quite possible that we tend to go overboard and let someone take undue advantage of us, which is not healthy.
Let me repeat: it's not healthy. I understand that you don't want your friend to struggle or fail an exam, for example, but you should never let a person control you and eat away at your time. Let me reiterate, it's your time, your life. If your locus of control is not you but someone who compels you to do endless favors that waste your energy, you need to take back control. Every time you help them, you just encourage this behavior over and over, making it harder for you to escape and stand up for yourself. Ultimately, this will spiral into you being depressed and overwhelmed — you have your struggles to deal with on top of constantly being asked to help the leech.
For example, I've had to deal with a persistent leech all my freshman year of college due to us sharing nearly the same classes and me having an overall greater understanding of the material. This is not due to intelligence; he needed help due to overloading on classes and being overwhelmed with his insane workload. It also didn't help that he sometimes never bothered to pay attention and just played on his phone/texted/slept during classes.
I helped him since I saw him as a friend, but it quickly spiraled into him invading my time and free space to work on problems that he put off until the last minute. At one point, I spent six hours teaching him the material for our differential equations final on that same day and realized just how much of an idiot I was.
This leads me into one of the greatest life lessons I've learned: don't feed the leech. Feed it and it grows; starve it and it detaches. If you make yourself less available to them, you will make them depend less on you. In extreme cases, you should voice your concerns to their faces so they are better aware of what they've been doing to you. If they still choose to be your friend, it will be on your terms now, and if they don't, that's a bridge worth burning.
Stand up to the leeches and take back what's yours — your time, energy, and sanity all matter much more than helping someone who is just going to continually use you. If you can do that, you can better handle any bullies that life throws your way, so you can evolve into a more confident, assertive person who doesn't take any crap from others.