I grew up with a mother that had this issue so strongly that she literally gave her car away to someone once. This way of thinking was drilled into me from a young age and I struggled very hard for a long time. I think I still have an internal struggle sometimes and I often want to offer to help out when I see I can do something nice for another, or that I know something that could be useful, but I do a lot of self talk these days.
As far as offering when I can see where I can be helpful, I try to remember that the person did not ask for my help to begin with, and they may take the offer of help to “be nice” or because they don’t know how to say “no”. I have had many situations where I have felt like my efforts were not appreciated, which in the end left me feeling resentful inside. But the truth is that many times they may not have had cause to really feel grateful, or not very much so, because of that kind of situation.
Also, I have noticed that there are “givers” and there are “takers”, or the “selfless” and the “selfish”. This is not meant to be a ‘negative’ proclamation, but rather as a foundation to say that I believe that some people are meant to learn how to be more “selfish” (in a healthy way) and other people are meant to learn how to be more “selfless”. To be too much of either is not a good thing. It is not the “right” thing. We need to have balance in all things, including this.
I will also tell you that the very best thing you can do is to learn how to say “no”. This is probably the most important word ever uttered in any language. To be able to say this with a calm surity rather than with anger, hatred, frustration, etc. To be able to say it straight forward without beating around the bush because you don’t want to “disappoint” someone. You are an important person as well. If it doesn’t work for you, the very best thing you can do is to say ‘no’, politely if possible, and not be guilted into something that isn’t right for you.
When you do this, you are caring for yourself, which means you won’t end up fostering negative feelings in conjunction with the thing or person you said yes to. This, in turn, will likely give you a better relationship with the other person. Also, believe it or not, many people will respect you more if you are able to say “no” when something doesn’t work for you.
In a world where considering yourself is seen to be a bad thing, it’s hard to do this. But, do not let this bring you down or think unfavorably about yourself. You have recognized that you do this. That is excellent in your progression. I urge you to sit down and really walk through the times you have done this and look at the situations from all angles. Try to find out what feelings pushed you into things, or what wording other people might say causes you to to feel those feelings which in turn lead you to acting accordingly. As you go along see if you can figure out where in your past these urges came from originally.
This will undoubtedly be very uncomfortable, and you may feel a lot of guilt and/or shame, but these are things you did in the past. They already exist, so you might as well try to learn from them. As you go along, there is nothing wrong with talking to yourself like you would a beloved friend. Comfort yourself like you would that friend. Tell yourself that you are working hard to become better and that those things from the past do not have to define you.
Love yourself enough to tell other people no. And love them enough to tell them the truth. Don’t commit yourself to things you don’t mean, or things you don’t want to do. You are allowed to say no. As you learn what feelings and situations fuel these things, I would recommend closing your eyes and pretending your in that situation. see yourself politely declining or not offering randomly to be “nice”. Bring up positive feelings that would support you in that moment and let yourself feel them. And let yourself feel the happiness and satisfaction afterward as if you had, indeed, gone through that situation in reality.
Your brain often doesn’t know the difference between that and something in reality. This is wonderful practice for the real thing. And don’t kick yourself when you’re not perfect at it all the time. You will get there. This is a journey. Let yourself go through the “Hero’s Journey” in your own story.
I hope all that made sense. It is late and I’m tired, but I saw your post and wanted to reply (which always ends up in some rediculously longwinded essay).
Best of luck. 
