Welcome to the forums- thanks for reaching out here, it can be a scary step, so go you!
Firstly, I just wanted to remind you that in strange times like this, having ups and downs with moods is perfectly normal and valid- don't feel disgruntled that these feelings are there. What's important is that you want to address this group of feelings so you can grow for yourself and partner.
Secondly, a good thing to do might be to address the root cause of this 'funk'- here, some professional help might be really useful. We can navigate this together if you like- just let me know.
Thirdly, in terms of dealing with the funk in the moment, a good idea would be to be transparent with your partner so she knows what's going on. She might be the best person to lift your spirits, and this will communicate that you trust her and feel ok being vulnerable with her- an important stepping stone in being able to express your love for her, which you say you struggle with during your 'flat' mood.
Fourthly, you mention you exercise, and that's a great form of self care! May I also suggest more gentle forms of self care- this might lift your mood. This could include trying some mindfulness, watching a movie, or cooking- anything that relaxes you.
Hope this makes sense and helps you somewhat. Keep us posted, if you like, Cheezle93.