No easy missions left on Borderlands?

I've played a few missions now and reached Level 13. However, I've only got 4 missions available at the moment, all of them are rated at Level 15 (Tough) or Level 14 (Hard).

Are there other, easier missions I can take on first or something?

Solution 1:

As you've not stated where you are in the game, nor what you've done so far I can only give general advice:

Your best bet is to try some of the level 14 missions, taking it slowly so you can gain XP as you go. Either that or just go exploring, you'll likely find some more missions else where.

Solution 2:

As a general advice, if you want to grind a bit, check the map for question marks, so you might find new areas that can give you more missions. In some areas there are NPCs with missions, etc.