i was in exactly the same situation as you for most of my life! my grandparents are spanish and so is my mum (she moved here when she was 4) and we have always lived in the same house. my grandparents have always spoken spanish to me and i can not remember ever being unable to understand it but i never ever spoke it cause i was too shy and i used to get embarrassed when they would tease me. in the end i took up evening classes, which i found quite easy but really they were just to give me more confidence and basically reinforce what i already knew in terms of grammar and vocabulary and stuff. and now i am less shy about speaking it cause my irrational fear is kind of dying off. i spent a lot of time thinking about why i was scared to speak it, and a lot of it was to do with embarrassement, but i just had to keep telling myself there is nothing to be embarrassed about. i am now studying spanish at uni and am quite fluent and much more confident. before i always used to be less shy speaking spanish to strangers than family members, dont know why. and when i started learning french at school i was really good at it (i think this is largely because of my knowledge in spanish) and had no problems speaking it, even when i knew what i was saying wasnt 100% correct. so maybe practise with people other than family members, who can often be judgemental and a bit patronising, i find. in my family anyway!
oh and it always used to irritate me when relatives would first laugh at me for my attempts at speaking it, and then upon noticing that i had gone quiet they would start whingeing "why dont you speak more spanish?". but take your time, build up your confidencce in it on your own terms and in your own time. now if a spanish speaking relative goes on at me i just point out their lack of language skills, since they normally dont have any.