Hi Jaynie,
First thing: good on you for having two children. We have one and that's enough! So I want you to know that I admire you

(Hopefully that makes you feel good)
The sex thing is hard. For me it was a mixture of massive(!) damage after Ferg's birth so I felt incredibly nervous, and or course feeling oh so tired- like you mention.
I think that sometimes we put pressure on ourselves about what is 'normal'- some couples talk about having sex several times a week, but for us it's not like that. Sometimes it can be a while, as you know, tiredness and time flies with little ones and then you realise it's been a while.
Things are getting better now though that Fergus is older, and I feel more confident again.
Do you do other things that are intimate too? Hold hands, kiss, massage, talk even.. all with no pressure for sex. I find this is really important.
DH and I are fine and dandy, and find that if we do little things like this, everything else follows in its own time.
I don't know if this was helpful in the least, but wanted you to know you aren't alone