There is a phenomenon called separation anxiety disorder that you can read about here. While I think your theory of why it is happening is a good one, there can be many features that can cause this reaction.
The essential feature of separation anxiety disorder is excessive anxiety concerning separation from those (in adolescents and adults) to whom the person is attached. This anxiety is beyond that which is expected for the individual’s developmental level. The fear, anxiety, or avoidance is persistent, lasting at least 4 weeks in children and adolescents and typically 6 months or more in adults. If yours has lasted for more than 6 months you may want to have an evaluation to properly diagnose this.
Children with separation anxiety disorder tend to come from families that are close-knit. When separated from home or major attachment figures, they may recurrently exhibit social withdrawal, apathy, sadness, or difficulty concentrating on work or play.
As you state your age as 18 you may want to connect with a therapist who practices Cognitive Behavioral Therapy (CBT). This particular form of therapy can help with coping strategies that can be effective in calming this concern. The ‘find help’ section at the top of the page can help you find a therapist in your area. There are also other forms of treatment you can learn about here.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral