You may be assuming that you’re only as worthwhile as your achievements, love life, attractiveness etc. If you link your self worth to these, and for some reason they take a nose dive, then your self esteem can plummet too. Or perhaps you take a long term dim view of yourself. Take a look at your worth and see what you can do to help yourself.
If you have a high self esteem then your measure of your worth is high. Conversely if you have low self esteem then your value of your worth is low. Take a look and see what you can do or what faulty thinking you may be employing in idenitifying your self worth.
This is a common thinking error which can lead to low self esteem. Take a look and see if you apply this to yourself.