Fear of abandonment always starts from deep priority of relationship. Life is filled with options but people often believe the "I CAN'T LIVE WITHOUT YOU" phrase which creates insecurities in relationship.
Emotional struggle follows, then another mild fear of heartbreak sets in.
From the start, make sure that you grow in love with your partner instead of falling in love. And even when you fall in love, watch closely: are we progressing? Is the love growing, stunt or decreasing? Note that a decreasing love is an ending love. So expect abandonment at anytime.
Some ways to overcome fear of abandonment in a relationship...
First off, avoid being attracted to unavailable lovers. Fear of abandonment is more tense when the obviousity and chance of being abandoned is clear. When it is most to be in a relationship with an unavailable partner, keep in touch and trust each other.
Assure your partner and believe that everything is the same in the other end.
You should value your relationship as much as possible but your best priority should be your career. The purpose of life is to live it, explore things to the utmost. Human relationship doesn't give you any chance to explore to the utmost while your career does.
Set a goal, regardless of your age you still have a future and a dream to make true. If you make relationship a life sentence you're indirectly connecting your partner's actions with your emotional circuits. Once the vow is broken, you're in pieces.
Disappointments and discouragements may come but let your desire be to improve yourself mentally and physically in order to be bold in facing your fears of abandonment.
Many fears of abandonment arose from fear of being alone. So increase your self-worth and build up your self-esteem. Trust yourself and recognize that it is fear and nothing else.
If you've suffered abandonment before, bad experiences can breed negative instincts. Today is better than yesterday and do not expect anything similar. Expect the best for yourself, but avoid LOVING bad relationships.
One last tips to overcome fear of abandonment in a relationship is to stop being too careful, just trust and be realistic. Do not nag, jealous or too clingy. It pushes people away, which might brought to the manifstation of fears.