With "just letting yourself be down," you have to accent the right words :-) Think about the idea of "letting yourself" instead of the idea of "down".

If you let yourself, what would you do?
If you let yourself, what would you feel or experience?
If you let yourself, how much rent would you charge

You can feel several things simultaneously; I bet you smiled at the last line? You were slightly amused or maybe puzzled if you didn't get it.

There are advantages to feeling down, think about what they are in your own case. When I'm down, I often feel like just lying in bed with a good book (and sometimes I have to first go out and find the book I "want"). I can like lazy days like that is I let myself. Why are you "disappointed" at being cool and gloomy? Like the Arabian proverb says, "All sun makes a desert". You like sun in your eyes, beating down on your head, and being too hot? Add other, more pleasing words to your list. Is it "quiet" too? That can be pleasant sometimes. Good for sleeping or rest. How's the humidity where you are? Is it good weather for a walk?

I was a bit afraid of lightning and thunder and my brother or father once taught a girlfriend who was spending the night and me to count to 5 to see how "far away" the lightning had struck (sound travels slower than light; every 5 seconds you can count, the lighning hit a mile further away/previously). That was helpful (we were about 10-11 years old) but I found in my 20's when I was living alone, I was still somewhat unnerved by it if it got to "furious" out there :-) So, one day (I lived in Washington, D.C.) during a bad summer thunderstorm, I went out walking in it! I carried on a conversation with my God and "defied" the elements, LOL. Just the experience and the remembering of that day helped a great deal. I did "something" for myself, trying to help myself and it worked!

Think of something to go with cool and gloomy. Meditate on them and find something amusing or interesting (gloomy reminds me of some movies or book settings, etc.) and make your day "special" to you so cool and gloomy don't mean something you don't want/reject in yourself? What does "a bit cold" mean? How cold is it when it's a "bit" cold? LOL, I'd get 10 washcloths and hold them under various temperature water/faucets and see which one was "a bit" cold for me. Play!