A picture of your affected fruit (and plant
) would be most helpful to fully understand what you're experiencing. Also a pic of the fruit sliced would be helpful as well, I think.
Based on the data you've provided so far, it could be a couple of things. You stated that your plant
isn't all that healthy and you're seeing lots of yellowing leaves. When that happens and the leaves start to wither, it increases exposure of the ripening fruit to the sun, which can cause sun scald. Sun scald is seen on the fruit as patches of whitish, hard bits. When one cuts into the fruit, the white bits extend into the fruit and appear kind of like an unripe apple. Another factor that can cause mushiness is over-watering or too much rain. Look for cracks in the skin of the fruit as well. We're having a big thunderstorm here today and the first thing I'll be doing when I get home is trek outside in my gore-tex and picking any almost ripe fruit before it can crack from absorbing too much water. If it's a rain thing, there's not much you can really do to counter that, but if it's sun scald, you might be able to provide some shade cloth to prevent too much direct exposure to the sun.
One last thing: it could be a combination of the variety itself combined with your environmental conditions this growing
year. I'm growing
an heirloom called Paul Robeson which had rave reviews of tasting deeply rich and complex. I picked my first one last weekend and it tasted watery, mushy, and tasteless. Needless to say, I was way disappointed. The good news is that it made a very tasty ketchup. So keep that in mind..even if you've got watery, mushy tomatoes, you can always spice them up in something cooked and they'll rock.