I fell one hand on july 12 and sprained my thumb. it is still hurting me.how can i tell how serious it is? it feels like my ucl and my rcl are hurt

Hard to say: Its been about 4 weeks since the injury. You mention joint laxity as a condition. At some point you may want to be evaluated by an orthopedic specialist. If the swelling has gone down you can get a good look at the ligaments and tendons with MRI. Management will depend upon your response to therapy and the structural integrity of the tendons and ligaments.

Orthopaedic eval: Since you have waited almost a month would recommend orthopaedic evaluation with Xrays and stress views of the thumb.