I am so sorry to hear of your situation and obvious distress.
First may I suggest you look and take comfort from how you have behaved since your offence; you have even shown your character's strength of community and social caring by working in the NHS when many of us would not of.
When reading my words please do not think I am trying to undermine your character in any way but more of support for you to realize you have the strength within you to face challengers, and importantly have the moral purpose to be honest to others.
Your words seem to suggest there are 2 major disclosure concerns: employment and your relationship.
Employment: I am not sure what type of role you are looking to move into and therefore would enhanced checks be undertaken thus disclose your offence? Others reading your post maybe more suitable to offer advice on your need to disclose after so many years or if I am correct in that unless the subject arises you do not have to raise it yourself.
Even if they do, would you not have excellent references from your NHS superiors concerning your character. I would suggest if the NHS have employed you for 20 years then others should not have issues even moral ones.
Relationship: I can understand but I am surprised your partner does not know. I know everyone has secrets but as my old gran used to say, "the truth always comes out in the end, be you here or not". By that she meant when you eventually depart someone will probably mention it, even if only to praise your partner for standing by you. Does your respect / love for your partner only exist for the "present" and so wish for them to find out that way; what sort of memory of you would that last for eternity.
Many poets write about in times of distress "letting someone go if you truly love them". I am sure you understand their meaning thus you can ask yourself questions such as -
"am I being selfish because I do not wish to be alone?"
" do I doubt my understanding of her/his character after 15 yrs and so not confident they would stay if they found out?"
If your answer is YES to either or both of the above then I would suggest your stress may be caused / added to because you doubt the strength of the relationship and what it is based on: honesty to and respect for the other.
The same can be said about the new role you are thinking about: you do not wish to be rejected because emotionally you feel you would go back to the feelings when you committed your offence. Believe me when I say you have proven you have the inner strength to be confident about yourself, so use that in the challenges you face because if you cannot be honest and respectful to yourself then how can you be to others.
Good luck with your challengers - know you are not alone when facing these issues or when seeking support.
Learn from yesterday, live for today, hope is for tomorrow else what is left if you remove a mans hope.