You are experiencing the symptoms of presbyopia. When you were younger, you were able to focus at all distances with the same prescription. As we get into our 40's and beyond, the focusing becomes separate at differing distances. That is, you need different prescriptions to see at far and at near. Your prescription for seeing far will no longer allow you to see at near. That is why you now need a bifocal to focus at near (with your glasses on). You can still see near without your glasses because you are nearsighted (but you can't see far without your glasses). Likewise, you can see far with your glasses, but not without them. In order to do both with your glasses, you need the bifocals.
I hope this helps and thank you.