This notion/idea of a 'younger self' exists only as a mental concept - which your physical mind currently identifies with and perceives as 'real' (having value/substance/relevance)...

There isn't any actual substance behind that thought-form and identification, though. Once you eventually make yourself aware of this, you will no longer feel like you've lost something - instead you will feel like you've gained something (awareness/knowledge).

Think about it... Who/How you perceive yourself to be has never actually been a fixed 'thing'... Your physical body itself is constantly changing and has been since birth. Your mental/emotional state is never a fixed 'thing' either. Thoughts and emotions coming and going - but never have any permanent presence within your state of consciousness. Your external life circumstances are constantly changing as well and have also never been fixed/constant.

This is why it's ultimately futile to strongly identify with one's physical appearance, race, social/economic status, professional status/reputation, external life circumstances, etc. (making a 'sense of self' out of this)... These aspects of the human experience don't last and have no staying power (permanence) . Sure it feels normal/natural to identify with these thought-forms for a period of time - but inevitably 'life' will lead you to a place where you have to let go of those attachments and identifications - and will prove to be liberating (freeing)... An opportunity for evolving (increasing one's state of awareness).

Perhaps there is something about your present life experience which is undesirable to you, overwhelming, and/or burdensome. Since you haven't figured out yet how to feel differently in response to your present life experience/circumstances (and this an be accomplished!) - your physical mind tries to cope by influencing your state of consciousness to identify with the imagined 'younger self' thought-form which represents a time in your life experience where whatever current conditions you are presently experiencing were non-existent... It's like an extremely temporary and superficial sense of 'release' that one might get from imagining and consciously reconnecting with an earlier version of one's 'self that was perceived to exist without one's current burdens/afflictions/hardships being present. I believe this is why people can and do enjoy the experiences of feeling 'nostaligia' so much... Things which remind them of an earlier time in their life experience. When they experience those nostaligic feelings, it's like they are temporarily 'disconnecting' from their current state/condition (and perceived sense of self). So they experience a very short-lived sense of relief/release... Does this ring true for you?

You can mourn the perceived 'loss' of your 'younger self' - until you reach the point where you realize that that concept (thought-form) did not really hae the substance/value that you once believed it to hold. And that very well may be the purpose behind you going through this experience now. To bring you to that important awareness and understanding. This feels real for now, and the sense 'loss' feels real for now. It's real for you until you bring yourself to a state where you have moved beyond thinking and perceiving (and identifying) in this manner - and then it's looked upon as unreal (illusory). To experience the sense of 'loss' is what it takes to push a individual in the direction of cultivating that important state of awareness, which will prove to transform your life experience...