Speaking as someone who has attempted to "deal with" severe PMS and depression by myself, I can tell you that it's hard hard work, and you don't get a medal for making yourself miserable.
I didn't want medication, so after a few years (!) I got help from a Dr specialising in this kind of thing (not in the UK, but your GP ought to be able to help / recommend someone who could help). I was given advice specific to my situation re diet, exercise, various non-medical therapies (yoga, counselling) and put on a specifc kind of mini-Pill which all really, really helped. I still had to deal with stuff, but it was a lot easier, and it was all without psychiatric medication. NB psychiatric medication can also be an absolute God-send, as several of my friends can testify, so I'm not saying you shouldn't try it if it's recommended for you and your situation.
This is not to say that this approach will work for you, but clearly "faking it" and trying to deal with it yourself isn't working so well - bouts are years long, and you're isolating yourself from your loved ones trying to cover it up. If you're covering it up and not expressing yourself, then it would be hard for your DP to understand that you are serious when you ask for help - my DP needed some very frank conversations before he understood how he could help me.
Good luck with it all, and I hope you can find the right path for you to get help.