Starting CON doesnt matter as much as people sometimes make it to be. The usual difference from the minimal con you usually start with to the average con people consider to be enough is 6, from say a minimum of 8 to an ok value of 14.
6 con will not make or break your character. Especially that those build points could have been spent on other, possibly more important stats. Neglecting every other aspect of HP will break your character though. So not having:
- False life items. GFL goes up to +30 hp and is fairly easily obtainable.
- +CON items. Equip a +6 CON neckpiece and bamm your initial -6 CON evaporated. Also fairly easily obtainable at higher levels.
- toughness feat! Must have. Especially if you have CON issues at build time, do take one (but not more!) Toughness feat. Preferably at character creation so you can enjoy the benefits from the get go.
- toughness enhancements. If you have taken the feat, you can buy these for a fairly cheap cost of APs at the trainers. Can go easily up to +60 HP with a double line.
- toughness item. Mostly Minos Legends is mentioned here, but toughness, a +20 stacking hp is now available on random gen items as well. Easiest and cheapest to find it on a cloak.
- elemental energy items, they give +10 +15 +20 hp. Originally when it was introduced it was only on green steel items, which are still a good (although expensive) way to pump up your HP, but now other items also carry this tag. Look into named items on higher levels.
- augment crystals, passive defenses, what are considered to be DR, will indirectly increase your survivability. Like invulnerability armor on low levels substracts 5 from each incoming physical damage. Stoneskin spell, stoneskin proc armor.
As you can see, there are so many things you can do to up your hp, even if you dont put points into CON at build time.