A judge called his behaviour "abhorrent" and warned the 20-year-old if he ever did it again he would be going straight to prison.
But Judge David Tremberg said he was able to suspend the sentence because Coggin, who was aged between 17 and 18 at the time, was of "positive good character" and pleaded guilty.
Prosecutor Charlotte Baines said that when the pair first met, Coggin would pick her up in his car and take her to McDonald's.
They began a relationship weeks later, and "at first things were okay", but then she began to notice changes.
He was "difficult" about her talking to male friends, and eventually, she had to block them on social media. She also lost contact with girlfriends and became isolated.
Coggin would not let her have a bath on her own unless he was "on the toilet seat watching her".
If he left the room before she had time to wash her hair, she had to leave too. Coggin would say: "If she had greasy hair, at least nobody at school would fancy her".
As well as stopping her brushing her teeth, Coggin also prevented the girl from wearing clean clothes.
Baines said when the girl told him this made her "feel dirty", Coggin said "he did not care, she was with him and did not need anybody else".
'He would talk about their relationship being permanent - they would get married and have children,' she said.
The girl was 16 at the time and would soon be going to college. Coggin would drop her off at home before making the girl FaceTime him at night until she fell asleep, accusing her of cheating on him if she didn't.
When she ended the relationship, Coggin "sprayed WD40 into his mouth in an attempt to stop her leaving him, but she had the courage to walk away".
In his police interview, Coggin, of Victoria Dock, produced a prepared statement and blamed the girl for causing "arguments".
He later admitted two offences of coercive and controlling behaviour, and was sentenced to five months in a young offender institution, suspended for two years. The maximum sentence is five years.
Defence counsel Paul Genney said the relationship was "obsessive" and "adolescent", adding that his client is ashamed of how he behaved and is effectively a different person to who he was then.
Judge Tremberg said Coggin's behaviour was "persistent, sustained emotional blackmail".
He told him: "If I were in your shoes, I would regard today's sentence as being your first and last chance, because if what happened to the complainant forms part of a pattern, it seems inevitable the next step for you will be prison."
Coggin is now subject to a two-year restraining order. He was ordered to undergo up to 30 days of rehabilitation, and attend a relationships course.
Where to get help:
If it is an emergency and you or someone you know is at risk, call 111.
• Women's Refuge: 0800 733 843
• Victim Support: 0800 842 846
• Lifeline: (09) 522 2999
• Family Violence Info Line: 0800 456 450
