Unfortunately, you are too heavy for that horse. If you follow the common 20% rule as in you should be no more than 20% of the horses weight, then you are definitely over the limit when you add in a saddle. Don't listen to anyone who says otherwise, some riders on this forum put their enjoyment over the health of the horse. We see it all the time, whether it's someone jumping a 2 year old horse or a 300 lb man riding a 900lb horse, it's cruelty anyway you look at it.
Google: "horse 20% weight"
You'll find a lot of discussion about the subject on various forums and websites. My advise would be to sell the horse and get a larger one.
Lots of discussion, ongoing, but as reliable as the drink 8 glasses of water a day, it's an arbitary number plucked out of the air by someone trying to set a number.
The hard truth is, you cannot put a set limit, because it does all depend, depends on the riders ability, weight distribution, balance, etc etc.
Depends on the horses build, bone density, and more than anything his heart and will.
Depends on correctly fitting tack
Depends HUGELY on what sort of riding you are going to do. A 20 minute walk around a paddock is a world away from a days hunting across country at full gallop and over fences.