Is Tendonitis Curable At The Age Of 18?

I'm an 18 year old male. i played alot of sports throughout my childhood and i have awful knees. i went and saw my sports trainer at school last year and she said i had bad tendonitis. i did the RICE technique and it worked for a little. But after not enough rest my knee is worse than ever now. it hurts even when walking long distances or just going up the stairs. im contemplating on calling a doctor but should i? the trainer told me that you just have to live through tendonitis. is she right? what should i do?