Hi Scotty, well done on looking for help.
From what you say it sounds like your partner is going to find out anyway. Better to come clean and tell them yourself.
Don’t just tell them about the problem though, show them what you are gong to DO about it. You are taling in £ so i am assuming that you are in the UK. You have lots of support available. You are welcovme here of course but also check out Gamcare and Gamblers Anonymous meetings in your area.
Using support will help you address the adiction, show your partner that you are taking positive actions to adress thigns and also, if it comes to any prosecuston, might be looked on kindly by the police/courts. If you aren’t going to be able to pay back the money in 2 weeks talk to your employer, you MIGHT be able to come to a different arrangement and again showing them you are using support for your addiction might help with that.
Forget “I feel i could have got the money back by gambling a little more” it wouldn’t have happened. And if it did the addiction would have just carried on and dug the hole again.
Read the other stories here, you will read a lot that you will relate to, at GA you will meet others who have done exactly the same as you have. You can stop gambling and you can move on from this. It is just important to use support.
Keep posting and maybe see you in a group here soon.