Losing control sitting next to yours truly (6,7)
I believe the answer is:
beside oneself
'losing control sitting' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'next to yours truly' is the wordplay.
'next to' becomes 'beside' (I've seen this before).
'yours truly' becomes 'oneself' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'beside'+'oneself'='BESIDE ONESELF'
Can you help me to learn more?
(Other definitions for beside oneself that I've seen before include "Overcome with worry" , "mad" , "Overwrought - is so enfeebled (anag)" , "Irrational, eg with worry" , "Distraught" .)