How do I keep my seedlings alive? I can get any seed to sprout and I can keep big plants alive but I cannot transition a seedling into a big plant. It's frustrating me, I don't understand the problem. (Tomatoes, herbs, trees)

yeah, me too.
I prefer to start all my tomato seeds together in a large pot... then, I don't have to disturb them until they're large enough to separate and set out.

What kind of trees are you trying to grow?

Are you giving in to the temptation to keep watering and putting them at risk of dampening off?
Are they spaced nicely so they don't develop fungi from lack of aeration and too much moisture?
Are they in a good position and not straining for light, elongating and weakening?
Do you put them out too suddenly rather than hardening them off gradually?
What's going wrong with them exactly?
Are you using any old compost or seedling compost (they don't need to much nutrition in the beginning)?

Chandra83 said:How do I keep my seedlings alive? I can get any seed to sprout and I can keep big plants alive but I cannot transition a seedling into a big plant. It's frustrating me, I don't understand the problem. (Tomatoes, herbs, trees)

I always direct sow my vegetables and herbs where they're going to grow to full height. For trees, I always sow one or more seeds into pots outside after the last chance of frost has passed, and this is whether I germinate them to sprout beforehand or not. I don't transplant trees until they've grown for three or four months. My experience with the trees is that I've grown Black Walnut trees from seeds (I only kept one as I intended to sell them locally), Apple (for rootstock), and Sand Plum.

I planted some jacaranda mimosifolia seeds and a couple other kinds of trees to bonsai, the sprouts were so healthy looking and then they just drooped over and never stood back up. I water them twice a day... my tomato plants are doing the same thing. Im using the soil that came with the bonsai seeds and seed starter soil for the tomatoes and herbs. Each seed has its own space in the container. Maybe too much water?

Way too much water.

When I looked up how to care for bonsai it said to keep the soil moist, guess it only meant until they sprout... oops. I'll cut back and see if the rest survive. Thank you all very much for responding.