Because a woman's identity is often strongly tied to her ability to sustain and nurture relationships, many women tend to be highly sensitive to and invested in the wellbeing of others. This can be a great strength and source of fulfillment, but it can also be a great liability and cause of emotional depletion. It sounds like the scale has tipped to the burden side for you, and as such, it's time to put some parameters around your giving. In order to preserve what's positive about your nurturing tendency, it's vital to learn how to be discerning about how you dispense it.
The first step is to look at why you feel like you have to be everything to everyone. Most likely it's because you're overly invested in being liked and avoiding conflict--again, a propensity that tends to be overrepresented in the female of the species. When your self-esteem is too dependent on how others feel about you, this puts you at their mercy.
You need to learn to be the captain of your own ship, to grow self-esteem based on self-care, self-respect and self-love. In order to do this, you need to decide where and when to draw the line of your giving and to shift your motivation for giving altogether. You shouldn't give solely because you want to be liked and fear being disliked if you say "no." You should give because it feels right to you and because you truly feel that you want and have the capacity to give. This requires a willingness to risk displeasing others, creating conflict or disappointment, and even the possibility that you might not be liked (gasp!). I know this is a hard concept to grasp, and an even harder one to put into practice, but the truth is that most people have more respect for someone who has a backbone and sets boundaries than someone who they may see as a walking doormat.
Start fleshing out who you are by acknowledging what you do and do not want. Let self-care be a good guide; retract from the reflexive impulse to give to others and first check in with yourself and your motivations. Do you really want to take care of your friend's yappy toy Poodle while she's away in Maui for the week, or are you just afraid she'll give you the heave-ho if you refuse her? I thought so! So, this means you're going to need to practice saying "no." While difficult at first, this will put you on more solid ground with yourself and others.
Remember, you're trying to build a stronger foundation for yourself and this is one of the first steps in that direction. The more you do it, the stronger and happier you'll be and the more room you'll have in your life to pursue your own desires. You might also find that your relationships feel stronger, not weaker.
I think we tend to sell others short when we sell ourselves short. So don't sell yourself short! Have faith that you can change this behavior and that others will respect you for it.