My horse has a heart problem...?

My 24 year old mare has a heart murmur. I found out about it this June and had to retire her early...so I couldn't ride her in the county fair...Now I'm just really apprehensive about riding her anyways...I know I probably can still ride her, but I'm afraid I'll wear her out or push her too hard...I've only been riding horses for nearly two years, so I know all the basics...I also know not to lope her at all or very much.....How can I get over the fear of "pushing her to her death"??? She was my first horse ever and she's an absolute sweetheart...so that kinda means I am really wary of riding her and all....