I have also felt used in the past as a result of being too kind and generous towards people with both favors and precious time. Even a no in disguise was guilted away and I felt like I had no choice but to push myself when I didn't actually have the energy to help.
Being kind and helpful are great attributes, and at the same time it is superb that you have adopted "no" into your vocabulary. It is not as easy as some may think.

Trial and error is probably the best way to go about it. Offer help where help is needed and don't be naive. If you value the person that asks you for favors or money, talk to them if you can about a long-term help instead of momentary. If they borrow money without returning it look into if they need help with an expensive addiction maybe?
If you can bare parting with a questionable friendship, then two strikes and you're out.

Rule of thumb to keep in mind is to not part with money or things you really need or cherish. A reasonable person will understand. Don't let good relations sour because of an unpayed loan.
At the same time we can not expect a favor for a favor, but without offering some type of reciprocation, it probably isn't a healthy relation worth keeping.
If you're not treated with equal respect and worth, then ditch 'ehm. In my opinion that is nothing but a toxic relation.

Continue being assertive and putting your foot down. Be kind when needed. What it means to be kind however is something only you can decide for yourself.
Be honest towards yourself and others about your limitations when it comes to energy and patience.
It's not human to have a neverending source of both.

I hope some of this can be of help!