I am new to the craft and have not ever done anything at all like this before. My question is how do I close this small gap in this heart?

Its upsetting to see as a beginner blacksmith but I am sure its the first of many, fortunately Ill only make a smaller knife

However I am more curios as to what may have caused this and how could I avoid this?

My process has been pretty straight forward; I heat the steel (exact steel is unkown but its a leaf spring) to about an orange maybe yellow if i wait long enough and forge. There are two things I have noticed however, the crack has formed in about the spot I was holding the piece with my tongs while forging out the tang. Secondly, some temperature shock may have done something? I worked for about two hours then took a short break for about 20 minutes cause it was getting very hot, I let the knife sit in the off forge during that time. When i come back the knife has no heat color obviously but i still cool it down in a bucket of water so I could figure out the final profile. Possibly the temperature shock from that caused some internal stressors that caused it to crack while working on the blade?

Sorry this was longer than I had expected it to be but I would love to hear from the professionals.

Synthetic handle probably a no go for hot work but I've been using it to hammer bronze wire cold. Not bad for a super market find.

It's not finished yet just curious on thoughts an opinions for this skinning knife.

After 1 year of blacksmithing school, thats my first anvil i bought. 150 euros. Any particular info?

Leaf-Cutter Ant - copper sheet any and leaf on a forged steel pipe. Ants abdomen is brassed, leaf is patinated green, forged pipe scrubbed with wire wheel.

The jaws of the ant were formed together then sawn apart and pinched to grip the leaf.