If someone has abandonment issues, they do not value themselves very highly. They constantly blame themselves for being abandoned. They believe that they are not good enough or capable enough to have people stay in their lives. Coupled with that is their self-pity which makes things worse. Trust me when I say that a low self-esteem and self-pity are the most dangerous of combinations to drag you down.
The fear of losing someone.
It is natural for a lot of people to have abandonment issues due to their unsettled past. That develops a fear of loss in their heart. They assume that at any moment in life, someone could leave them and just disappear without giving any proper reason or justification for it. As a result, when they are in a relationship, you will see them constantly under the fear of losing the person. They will behave oddly and sometimes, repel their partner as well because not a lot of people understand what it is like to be with someone who has faced abandonment in their lives.