[Closed] Serious concern effecting my riding

Hi everyone. Firstly I am a average to ok rider.

Right I have been seriously struggling with energy whilst riding! I start off just like normal 10 minutes to get warmed up. I then seem to go fine for another half hour or so and then bang it's all gone. Today it was so bad I had to get off and sit a minute to recover, it's only swinley so a bit pathetic for me.

It's happen the last 3 rides I don't get it. It's not like fitness it's more like being 90.

Has this happened to anyone else ? How did you fix it?

I fully intend to go to GP but as they are closes till Monday thought I would ask the wise?

Thanks