HI there, I was the same and as a child I had so many terrifying experiences that I shut my psychic sense right down. I know now that this was because I was in a terrible place mentally as a child, and so that's how I interpreted my experiences, with no onw to tell me any different. That and I watched too many horror movies
Doing something like reiki or something like meditation would help you reconnect with the spirit world. Ask that your spirit guides and protectors aid you and protect you, either out loud before a meditation, or in you head. Be very specific, 'i ask my spirit guides to aid and protect me as I do this meditation / whatever.' Take it slow. There is nothing to fear, you get out of mediumship what you put in, iykwim.
There are also lots of 'guided journeying' CDs on the market - check out amazon.co.uk.
You can get these for spirit work or shamanic journeying, whatever.
I was in denial for so many years, and terrified of what might happen. I am in a much happier place now, and my gift has come back of it's own accord, and it is the best thing that has ever happened to me.
I have had visits from every member of my family that I was close to that have passed over, I know that they support me and are with me always. I have had messages for other people, this freaked me out a bit at first as I thought I was losing it, but as the messages and the information I got from the spirit (although I am not able to get much yet) has all been very accurate and comforting for the person who has been bereaved.
To reassure me, my Mil (passed over) said some things to me in spirit, I then went to a tarot reader / medium and she repeated what Mil said, word for word, from her. I knew then at least I wasn't totally crazy.
Today, in the media, ghosts / spirits are always portrayed as bloodthirsty, dangerous, malevolent, but the truth is that the vast majority of spirits are anything but. Don't worry, you can't 'force' contact on your grandmother, she will in all likelihood want to come through to you in love and reassurance.
Oh and ignore the people who say you're nuts, and contact your local spiritualist church. Be wary of phonies and don't get overcharged (or charged at all most of the time) for services.
Give us a shout if you need anymore rambling from me help.