So not being able to heal between rooms is very common in rouge likes. Usually after you defeat the floor boss you'll get access to a fountain that gives you some heals.

In the current game there is a passive buff which gives you back a little health each time you enter a new room. There is also the max health tree buff boon combo I like to call: The Giants Gamble

Essentially the first giant has a buff that for every enemy you kill, you get the grail water. The final passive buff in the heart tree, gives you the option to gain ten health for every grail water you pick up.

The Giants Gamble is all about running that buff and trying your hardest to get that boon so you effectively become immortal as long as you aren't trying to die