I don't know how much i have understood, or even misunderstood here... But i'll just note what i heard when i read it...

I'll quote what the writer of a TV series called Touch mentioned : "We recognize ourselves in each other, and are programmed for compassion, for heroism, for love. And those things make us stronger, faster...and smarter. It's why we've survived. It's why we even want to."

Every person loves to know they are needed, and can make a difference to the people they are surrounded by, care for... and if not, find someone they can care for and help...

However, you need to do that, by helping when required. By doing it without being self-sacrficial.
Putting their needs before your own, depends a lot on the person, their requirement, and how much and what you are compromising for it. It needs to be balanced.
And when you try to balance it, you will end up putting certain needs of yourself before theirs, and that is perfectly fine.
And if it is a healthy relationship, discussing this helps i suppose? Thinking of ways you both can be there for each other?

And for this, you will need to start seeing yourself as equally worthy of the support you give them. Your self-worth depends more on what you think than the people around you. You reflect what you think, and that is what they usually tend to believe. All upon your sense of worth of yourself. So learn to love yourself first.

Also need to realize that if you cannot help someone because of your own needs, it does not always lead to rejection, if that person truly respects and values you for who you are.

In any relationship, there should be both, give and take.

But because you have always been the rock for the other person till now, unless you feel ready to open up, and discover how to bring that to the other person's notice, i guess it is difficult for that person to get that when you are in need of his/her emotional support for yourself.
Once you start opening up, they'll realize you need them too, and i am sure if they respect your efforts to help them, they will be more than happy to help you back if they can.

Also do you find yourself being manipulated into guilt if you are not able to be there for the other person irrespective of your feasibility? People who do this are toxic people and need to be handled differently.

I don't mean to offend, but do you feel you are trying to compensate for something you feel ashamed of, by being the rescuer for others? I ask this as this quite a common reason for people being codependent on people in need of help.