Bev,
As you may have gathered by now from all the responses and messages of support, this is not necessarily a simple issue and there may not be one single thing that fixes it for you. Every person is different, and it may take a number of things in combination to set you right. I speak from experience, as I've been dealing with this for years. Your description of the October tunnel, April release sounds so familiar. And we have a very different winter than you do in the UK - it usually snows in mid-December and doesn't melt until early April. We've often skied on Easter week. We probably see more sun than you do, however.
Here's what I can suggest, based on what's worked for me:
+ Exercise! Outdoors! Most people see winter as a time to hibernate indoors, and then wonder why they feel blah all the time. Force yourself outside for whatever gets your shoes laced -- hiking, cycling (even in the snow -- get a fatbike), skiing (XC or downhill, depending on where you live), jogging (preferably trail running in the woods). If you can't get outside because it's raining for two weeks straight, then look to a gym and do whatever you fancy -- yoga, treadmill, squash, basketball....preferably something social so you can talk to people. Believe me, the endorphine boost from rigorous exercise will do wonders and lasts for days.
+ Take up a hobby that requires brain power and focus, preferably that gets you outside. Photography is my thing - black and white, film, darkroom printing, fine art stuff.
+ Change your diet if you eat too many carbs and red meat. Eat more fresh veggies and fruits, lean meat and fish, legumes like dried beans and lentils -- food that will give you energy rather than sucking it out of you. Cut back on white flour stuff like breads and pasta.
+ Cut back on refined sugars! Except at Christmas time, of course. If you need something sweet, use natural sweeteners like honey.
+ Lose weight, if you're carrying too much. Self-explanatory, but you'll have way more energy.
+ Vitamin D supplements -- yes, these do seem to work. This is the first winter I've taken them, and I feel noticeably different.
+ Take natural pro-biotics to improve your gut-health. This one can be huge, and a life-changer. There is a lot of serious and legitimate research into the connection between gut flora and mental alertness and physical well-being. Even major govt. agencies like the CDC/NIH in the US are acknowledging that probiotics like kefir and kombucha can have significant benefits. Personally, I tried this a couple of years ago and started drinking a cup of natural plain kefir every night -- within a week my energy level felt like it had doubled. I was blown away. Seriously, read up on this and try it. It's inexpensive and is so good for you. If you don't feel better within a week I would be surprised. (BTW, if you start drinking kefir or kombucha, your appetite for sugars may totally dissappear -- it's a neat side benefit).
+ Stress reduction. Again, a big one. If you have a lot of stress for whatever reason, all that other stuff that drags you down in the winter just makes it worse and it becomes a vicious cycle. Try your best to reduce the stressors, and manage those that you can't reduce by whatever means works for you -- yoga, meditation, exercise, music(!)
I really hope you can tackle this and get over it. It's too much of your life in the doldrums not to do something about it. Hopefully my little bit of advice will help you.
All the best,
Svend