Terrified of being left, forgotten, abandoned. engaged. both severely insecure and over confident. lack sense of identity, personality wise. help?

Speculative: Sounds very hard. A reaction to getting engaged? If long standing, it might be related to a mood disorder, consequence of family environment being problematic, a variant of a post traumatic reaction to childhood experiences, a personality condition (e.g. borderline PD), or possibly related to a medical condition or substance use. I'd talk with a counselor, and don't over-react but don't ignore it

See a psychologist: You should get evaluated. Schedule an appointment with a psychologist for an evaluation. Therapy and/or medication will help if in fact you are depressed or have chronic feelings of emptiness. Make sure you talk about your symptoms and not a diagnostic label.

Fear of abandonment: Impulsivity, lack of identity, and Lo self esteem or shifting self esteem are symptoms or traits that may be associated with borderline PD, although diagnosis should be deferred until one can be made by an MD who has met & evaluated you in person. What's most important, is that these feelings must be unpleasant, at the very least. Speak to your PCP or a psychiatrist to get help & feel better!