It might come as a surprise to hear that some of your struggles as an adult might stem from your childhood. Your upbringing, specifically, the part where you were developing your bond with your parents and learning to handle your emotions, might be the start of your unhealthy responses today. This isn't the time to pick up the phone and angry text your parents and blame them for all your struggles.
Before I go into the symptoms of separation anxiety, know that this is not only a childhood disorder anymore. In fact, research is showing that mothers get separation anxiety (i.e. maternal separation anxiety) from their children. Adults can have separation anxiety from one another too. This type of anxiety can be difficult to understand if you aren't familiar with attachment theory and bonding. That's a topic for another time, but feel free to do your own internet search, if you're curious.
When we grow up in an environment is that does not provide healthy attachment styles, we can develop fear of separation or separation anxiety. Unhealthy attachment styles can be traumatic, disconnected, unloving, or overbearing. Think of attachment as a specific path that must be walked by parents with their children. When that path is obscured, damaged, obstructed or dug too deep then the attachment can be confusing or distressing for the child(ren). If attachment is important to you, I suggest reading more about it here.
Find more books on separation anxiety below.
Anxiety symptoms can present in many ways and not all people respond the same to their anxiety. This means that the symptoms of anxiety are wide and it's not a "one size fits all" series of symptoms. Anxious people might feel the following (not an all inclusive list):
Fear of abandonment by a loved one or others.
Fear that the loved one won't return because of death or illness.
Fear of being cheated on or being unloved.
Fear that the loved one feels unloved or not giving them enough attention.
Sadness when left alone from loved ones.
Sadness when loved ones don't respond to calls/texts (which lead to fears that they are hurt or dying.
Feeling upset or irritated when your loved ones do not reciprocate affection.
Experiencing nightmares about loved ones dying or leaving.
The need to constantly check on loved ones whereabouts.
While several of these symptoms fall under the anxiety category, these symptoms can be associated with other disorders. If you are experiencing some of many of these symptoms, please contact a mental health professional for guidance. You are not alone and these symptoms are treatable. If left untreated, these symptoms can cause difficulty in day to day functioning or worse.
Understanding attachment theory, parenting styles, and emotional regulation can help to explain adult separation anxiety. The way your parents and caregivers handled your distress affected how you handle your distress today. The way your peers and siblings responded to your intense feelings affected how you manage your intense feelings today. If you want to learn more about these concepts, I have provided several books for you to explore.
Nothing can replace the processing and healing that can be done during individual therapy; however, books are a good place to start.