YOU ARE ALLOWED TO SAY NO... but you may need some help getting started if this is difficult for you. It all boils down to the concept of self-worth and believing that your time is of value to YOU. When you are constantly saying yes to everyone else and putting yourself at the bottom of your own priority list, you will be the loser in your own life.
Here are some tips to get started.
Rather than thinking about saying NO to others... re-frame it to think about saying YES to yourself. This simple switch will reduce the guilt trip. The moment you take yourself more seriously and value your own priorities and goals to achieve each day, others will start taking you more seriously as well. You teach others how to treat you, so if you want a different result to always feeling like the doormat-take different action
SIMPLY - get over yourself! Harsh but true. Stop thinking that you need to be all things to all people. Whilst this seems wonderfully generous, you will come off second best and land up having nothing to offer. This will lead directly to the idea that you do not value yourself in the equation, and every time you say yes to others when you really ought to be saying no, you will land up feeling resentful, angry, used and taken advantage of. So give up this need to feel bad about yourself and to make you feel needed and important through being everything to everyone and nothing to yourself.
Use a buffer line as a standard response from now on when still learning the art of saying no. Try this out for practice... when someone asks you to do something, you can say; "I would love to help you out but I need to check my diary and I will get back to you tomorrow". That way you do not say YES as your standard answer, and create a pause for you to assess if you want to do it or not. Be aware you normally say YES out of habit.
Never say an outright NO as this can feel horribly foreign to you and rude to the other person. So always start your answer with "I would love to but am unable to right now". And remember that you do not ever need to give reasons or excuses as to why you cannot do something for someone else... give up the need to justify your life.
Have patience with yourself. Just because you want to say no doesn't make it easy!. Remember you are learning a new skill that's foreign to you. It can help to acknowledge the following - "even though it is really difficult for me to say no without feeling guilty, I am going to value myself enough to say no anyway" This can help you remember you are chartering new territory and the moment you do NOT expect it to be easy every time, it will help you keep moving forward. In no time at all it will become second nature for you to discern when to say yes and when to say no from your inner guidance as opposed to guilt and habit.
So go out and practice, practice, practice, and let me know how you do with this new skill!!