How to politely stop contact with someone you dont want a friendship with?

At school I had no friends for a while as my friends didn't stay on for A-level. I used to chat to a girl in my class but I wouldn't say we were friends. We never arranged to meet up outside of school. She wasn't someone I clicked with. She's painfully shy, and when she speaks she only talks about boring things. Anyway after three yrs of no contact she asked me if I'd like to go for coffee. I figured she might be a different person now so agreed. Big mistake! I am a big talker and love to socialise but from the moment she. arrived I knew I was in for a long afternoon. The conversation flowed okay but only because I had to make a HUGE effort to ask questions and the conversation was dull. After 90 minutes I said I had to get back. Now I was hoping it was as bad for her as it was for me but she has invited me for coffee again. I thought I'd be kind as she has no friends and agree to go with her (although I changed it to cinema as then I dnt have to talk much to her) but the cinema trip is next week and I'm dreading it. I just don't want to go. Being in contact is causing me unnecessary sadness and by her spending time with me I'm stopping her meeting someone who will genuinely like her. I tried to get another friend to become friends with her but they say they don't want to. I can't tell her I don't want to be friends as she's too sensitive for that. How do I put an end to all this?