So OP, what do you think you can do to improve your life from here onwards? Do you think it's possible to have an enjoyable fulfilling life without marriage and kids? Bearing in mind that doesn't mean you can't find a great relationship with someone and have the joy of children in your life by being close to and an auntie figure to your friends' kids, volunteering with children if that's your thing. How do you want your life to look if you remove marriage and your own kids from the frame?
One thing I find useful is to sit down and go through this www.div12.org/wp-content/uploads/2015/06/Valued-Living-Questionnaire.pdf
It's a questionnaire about your values, what's important to you, and how in line with your values your actions are. If you value something deeply yet you dont prioritise it in your life, or you act incongruently with your values, you'll feel very unfulfilled and low.
So have a think about how important to you those areas are, and whether you're acting in line with your own values. Do you feel that having a relationship is important to you, but you're not doing much to work towards it? Perhaps friendship is important to you but you're social life is stale and boring to you now. Maybe you don't really feel work and a career is too important to you beyond a wage but the majority of your time is spent on it etc.
Decide how important each value is to you and how satisfied you are with that area of your life at the moment. And then get to work on the areas where there's a big gulf between the two.
Decide on a few realistic achievable goals for those areas and plan out the steps you need to take to get there. And then get cracking.
It'll give you a sense of control over your life and hope for the future breaking out of this rumination and wallowing/defeatism. And I often find doing this helps me remember that there is a lot more to life than just the one or two areas I'm fixated on.
You won't feel differently until you start to act differently. Maybe this is a framework you can use to give yourself that kick and make some positive changes for the better?