If that's a crack in the seat tube above the Top tube weld,your frame is golden brown, but may not be toast yet.

First of all, understand what is causing it, which is excess stress from an unsupported seat post. The minimum insetion mark on the pot relates to the posts requirements. However, there's a second requirement that the post extend to about 1" below the top tube. On a frame with a design like yours this may mean that the minimum insertionis deeper than the mark indicates.

Also, an undersized post can cause a similar problem by rocking within the tube. If all is right, the post fully supports the seat tube where yours is cracked.

If it isn't cracked all the way around, you might extend the life with a longer, better fitted post, however the crack will probably continue to grow. If you're willing to sacrifice a post to save the frame, and you have no plans to ever adjust the saddle height, you can get a longer seat post and bond it in place with epoxy, or an anaerobic (loctite) adhesive. Once a strong post is bonded in it will hold the frame together through the cracked area.

Given your weight, it may not be a forever repair, but if done right will last a long time.

If you replace the frame, consider a careful fit for the tallest frame you can manage properly. This will shorten the unsupported post reducing stress at the base. Also a frame design with a minimum extension of the seat tube above the joint will prevent this kind of damage.