Every relationship is like a scale - one of those old ones, with a tray on either side. And certain experiences from the past might have made you believe that a relationship only "works" (and that you can only be safe and loved) when the amount of effort you put in makes your tray drop straight to the floor. The thing is, when you just keep on giving while putting yourself last, you are not in a position to receive. This may seem pretty harmless, and also pretty impossible to change, but nothing could be further from the truth.
Putting yourself last causes disconnection
In a sustainable relationship, both trays are up in the air. And no, they do not have to be at the exact same height at all times; it's perfectly normal for one person to give or receive a little more than the other for a while. But both parties should be given the opportunity to, through both giving and receiving, contribute to the relationship.
Putting yourself last causes disconnection; you are preventing the other person from putting something in their tray. For example, when you do not communicate what you want or need, you deprive the other person of the opportunity to be there for you and, if they want to, to try to meet your needs.
A relationship is like a dance
Instead of a scale, you can also think of a relationship as a dance. If you are always giving in to the other person and don't take up your own space, you are just following the other person around on the dance floor - and that is not a partner dance, that is a polonaise. If you're not fully present, your partner can't sense you or hold you or help you take your next step.
Partner dancing is about tuning in, inviting, taking steps and responding. By shrinking yourself, you take a step backward, you disconnect, and consequently, the dance will come to a halt. The only way to get this dance going again is to step forward - in other words, to take up space, to communicate your needs and boundaries, and/or to express your opinions. This is what’s needed to re-establish your presence, to resume the dance, to reconnect, and, finally, to jump back into each other's arms.
Taking the first steps to stop putting yourself last
To stop putting yourself last, you might try to share more often with your partner what makes you feel good and what doesn’t - even if you think your boundary or need is undue or too much. They are free to choose not to meet your needs, but it is up to you to give them that chance.
Something you might also give a go, is to stop giving just for the sake of giving (without actually knowing if it will make your partner happy). Stop just automatically taking on chores around the house, stop trying to please them by tuning in with their mood and adjusting your behavior accordingly. Those are all things people pleasers like us are really good at, but by no means are always appreciated by the other person.
People are in a relationship with you because they want to connect with you. And that's not possible if you become some kind of (energetic) extension of the other person, or are constantly fixing all of the things on your own.
Give your partner the most precious gift you can ever give them: you
You will encounter some bumps in the road during this journey and you'll have to tell yourself many a time, perhaps with a hand on your heart (or somewhere else on your body), that although you may not feel safe, it is safe for you to experiment with putting yourself first more often.
On this courageous journey, keep reminding yourself that although it probably feels like you are taking something away from your partner, you are actually giving them a gift: your complete, authentic self. And that is the only thing someone who loves you will ever really want from you.