My first step is to ask myself, "is there something they did that is making me feel rejected/forgotten/hurt?" if yes, I discuss that thing with the person: I ask them to explain the reason behind their action, because without explanation I may think the reason is lack of love. When they explain, I choose to accept their reason as the whole truth. If their actions are not the issue, or if I still feel bad, I ask myself,
"have I tried all my methods of self-care?" if no, I do whatever I can* (it is important that this step comes before the next one!), and if the feeling is still there I ask myself,
"how could this person make me feel more loved?" and then ask my person to do one of a handful of specific things** that would help me feel loved. If they do not want to do or do not feel capable of doing any of them, then I ask myself,
"is this a pattern or just a moment?" and if I can think back and realize that in another moment like this my person has made me feel loved, I can use that memory to make myself feel more loved (which is why I keep a log of love-memories!).
If on the other hand, this person does not ever respond to direct requests with taking loving action, I take a step back from the relationship. A person who is rarely/never capable/willing to make me feel loved is not a person who is safe for me to invest in; I have a finite amount of energy and I must invest it where I can create a mutually beneficial dynamic. Showing care in a way that translates to the other person as love is important; feelings without actions are not nourishing for the people being 'felt' about. If I want to be healthy and able to connect and give, I must be willing to press pause or stop on a relationship that is not nourishing me. I completely support someone pausing or stopping a relationship with me as well, and would welcome them right back if we got to a point where we could be mutually nourishing again.
And for me, if all of those immediate issues have been fulfilled and I still feel worried, I consider long-term issues, like chemical depression or a lack of daily investment in knowing and appreciating myself. Sometimes I need to have a self-date (or invest more in myself in general) and it isn't until I start feeling this way that I realize I have not been paying attention to myself.
* my "rescue remedy" self-care includes making sure I have eaten (my mood drops drastically when I haven't eaten in 5+ hours), watching a silly show, drinking coffee/tea, drinking plenty of water, and/or laying down and focusing on my breathing. If it's a mild bad feeling, going out to nature or to get coffee helps, but that also takes significant energy so it is not always possible.
** my specific "rescue remedy" care-from-others are giving me a foot rub, going out just to get me a latte and bringing it to me (or otherwise going out of their way for me), cuddling and kissing me while fully present, looking me in the eye and telling me what they value about me, lightly petting me from neck to ankle, and taking me out to nature.